[Docs] Fix headline levels and some spelling

docs/admin/01_core_concepts.rst

-The AlekSIS core
-================
+Concepts of the AlekSIS core
+============================
 
 The AlekSIS core provides functionality and data models as a base for
 all apps.
@@ -52,13 +52,13 @@ information like the following:
 -  Primary group (e. g. a class or a tutor group, cf. [@sec:Groups])
 
 Except for the name, all data points are optional, so you can decide on
-your own (and on your local data protection laws) which data should be
+your own (and based on your local data protection laws) which data should be
 included in AlekSIS.
 
 There are two important things you should know about persons:
 
 -  **Persons are not automatically users:** That means that persons can
-   be linked to an user account including things like a password and the
+   be linked to a user account including things like a password and the
    ability to login, but they don’t have to be. For example, your
    AlekSIS instance could save the data about parents, but you don’t
    want them to login: In this scenario, the guardians are available as
@@ -111,5 +111,5 @@ Import school terms, persons and groups from other data sources
 ---------------------------------------------------------------
 
 When AlekSIS is not your single date source, all these data can be
-imported from other sources. You can find further information in
-[@sec:Integrations].
+imported from other sources. You can find further information in the
+respective integration apps.

docs/admin/03_ldap.rst

@@ -20,7 +20,7 @@ very straightforward under all circumstances. On Debian, install these packages:
 Configuration of LDAP support
 -----------------------------
 
-Configuration is done under the ``default.ldap`` section in AlekSIS’
+Configuration is done under the ``ldap`` section in AlekSIS’
 configuration file. For example, add something like the following to your
 configuration (normally in ``/etc/aleksis``; you can either append to an
 existing file or add a new one)::

docs/admin/05_monitoring.rst

@@ -27,10 +27,10 @@ Thresholds for health checks can be configured via config file
 Status page
 -----------
 
-AlekSIS status page show information about the health of your AlekSIS
+AlekSIS' status page shows information about the health of your AlekSIS
 instance. You can visit it via the left navigation bar (Admin → Status).
 
-The page show information about debug and maintenance mode, a summary of
+The page shows information about debug and maintenance mode, a summary of
 your health checks and the last exit status of your celery tasks. This
 page can not be used as a health check, it will always return HTTP 200
 if the site is reachable.
@@ -65,13 +65,10 @@ Performance monitoring with Prometheus
 --------------------------------------
 
 AlekSIS provides a Prometheus exporter. The exporter provides metrics
-about responses and requests, e.g. s about response codes, request
-latency and requests per view. It al provides data about database
+about responses and requests, e.g. about response codes, request
+latency and requests per view. It also provides data about database
 operations.
 
-Metrics endpoint
-~~~~~~~~~~~~~~~~
-
 The metrics endpoint can be found at
 https://aleksis.example.com/metrics. In the default configuration it can
 be scraped from everywhere. You might want to add some webserver
@@ -84,7 +81,7 @@ To get metrics of your AlekSIS instance, just add the following to
 
      - job_name: aleksis
        static_configs:
-         - targets: ['my.aleksis-instance.com']
+         - targets: ['aleksis.example.com']
        metrics_path: /metrics
 
 Rules for prometheus alertmanager
@@ -110,7 +107,7 @@ is slow or something.
 Grafana dashboard
 ~~~~~~~~~~~~~~~~~
 
-There is a Grafana dashboard available to visualize the metrics.
+There is a Grafana dashboard available to visualise the metrics.
 
 The dashboard is available at
 https://grafana.com/grafana/dashboards/9528.

docs/admin/06_configuration_options.rst

@@ -63,6 +63,6 @@ Example configuration file::
 Configuration in frontend
 -------------------------
 
-Everything that must not be configured before the AlekSIS instance fully starts can be configured in frontend, such as site title and logo.
+Everything that does not have to be configured before the AlekSIS instance fully starts can be configured in frontend, such as site title and logo.
 
 You can find the configuration options in your AlekSIS instance under ``Admin → Configuration``.

docs/admin/06_storage.rst

 Storage
-##########
+=======
 
-Amazon S3
-*********
+Amazon S3 (or other S#-compatible storage)
+------------------------------------------
 
 AlekSIS allows you to configure an Amazon S3 endpoint for static and media
 files. This is useful e.g. for loadbalancing with multiple AlekSIS
 instances.
 
 Configure an S3 endpoint
-=======================
+~~~~~~~~~~~~~~~~~~~~~~~~
 
 If you want to use an S3 endpoint to store files you have to configure the
 endpoint in your configuration file (`/etc/aleksis/aleksis.toml`)::

docs/appendix/00_index.rst

-
-================================
+Appendix
+========
 
 .. toctree::
    :glob:

docs/dev/00_index.rst

-Development
-===========
+Development of AlekSIS apps
+===========================
 
 .. toctree::
    :glob:

docs/dev/03_run_tests.rst

@@ -6,8 +6,7 @@ Running default test suite
 
 The test suite can be run using the `tox` tool::
 
-  poetry run tox
-
+  tox
 
 Enabling Selenium browser tests
 -------------------------------
@@ -32,7 +31,7 @@ Selenium tests are enabled if `TEST_SELENIUM_BROWSERS` is non-empty.
 
 To set variables, use env to wrap the tox ommand::
 
-  poetry run env TEST_SELENIUM_BROWSERS=chrome,firefox tox
+  TEST_SELENIUM_BROWSERS=chrome,firefox tox
 
 
 Using a Selenium hub on local Docker host
@@ -50,10 +49,9 @@ First, get Selenium Hub and one or more browser nodes up and running::
 After that, you can run the test suite, setting the needed variables to use
 Docker Hub::
 
-  poetry run env \
-    TEST_SELENIUM_BROWSERS=chrome,firefox \
-    TEST_SELENIUM_HUB=http://127.0.0.1:4444/wd/hub \
-    TEST_HOST=172.17.0.1 \
+  TEST_SELENIUM_BROWSERS=chrome,firefox \
+  TEST_SELENIUM_HUB=http://127.0.0.1:4444/wd/hub \
+  TEST_HOST=172.17.0.1 \
   tox
 
 The `TEST_HOST` variable is set to the Docker host's IP address, where the

docs/dev/04_materialize_templates.rst

 Materialize templates
 ======================
 
-AlekSIS frontend uses the `MaterializeCSS`_ framework and the `django-material`_ library.
+The AlekSIS frontend uses the `MaterializeCSS`_ framework and the `django-material`_ library.
 
-Internationalization
+Internationalisation
 --------------------
 
 Load the ``i18n`` template tag and start translating strings in templates with

docs/dev/06_merging_app_settings.rst

@@ -25,5 +25,5 @@ the following into your ``settings.py``::
         }
 
 If you install new apps and want to configure these, or need some other settings you can easily add
-settings to your ``settings.py``.  Only settings that does not exist in the
+settings to your ``settings.py``.  Only settings that do not exist in the
 main ``settings.py`` will be respected.

docs/preamble/00_index.rst

-
-================================
+Preamble
+========
 
 .. toctree::
    :glob:

docs/preamble/01_introduction.rst

@@ -15,7 +15,7 @@ basis.
 On School Information Systems
 -----------------------------
 
-School Information System, or for short, SIS, are software suites that
+School Information Systems, or for short, SIS, are software suites that
 help educational organisations with managing personal information about
 all members (e.g. students, teachers, parents and guardians) as well as
 organisational data like plans, pedagogic track records, and much more.

docs/user/00_index.rst

-
-================================
+End-user documentation
+======================
 
 .. toctree::
    :glob:

docs/user/01_personal_account.rst

 Managing your personal account
-------------------------------
+==============================
 
 Each logged in user has several options to provided through the AlekSIS
 core. Which of these items are display depends on whether the user has a
@@ -8,7 +8,7 @@ person and what your system administrator has configured.
 .. _sec:Notifications:
 
 Notifications
-~~~~~~~~~~~~~
+-------------
 
 The AlekSIS core has a built-in notification system which can be used by
 apps to send urgent information to specific persons (e. g. timetable
@@ -19,7 +19,7 @@ channels. These channels can be switched on or off in your personal
 preferences (cf. [@sec:PersonalPreferences]).
 
 Setup two-factor authentication
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-------------------------------
 
 
 .. image:: ../_static/2fa.png
@@ -43,7 +43,7 @@ To disable two factor authentication, login to your account and navigate to
 `Account → 2FA`, then press the big red button to disable 2fa.
 
 Change password
-~~~~~~~~~~~~~~~
+---------------
 
 If your system administrator has activated this function, you can change
 your password via ``Account → Change password``. If you forgot your
@@ -52,7 +52,7 @@ helps with resetting your password. The system then will send you a
 password reset link via email.
 
 Me page
-~~~~~~~
+-------
 
 Reachable under ``Account → Me``, this page shows the personal
 information saved about you in the system. If activated, you can upload
@@ -61,7 +61,7 @@ a picture of yourself or edit some information.
 .. _sec:PersonalPreferences:
 
 Personal preferences
-~~~~~~~~~~~~~~~~~~~~
+--------------------
 
 You can configure some behavior using the preferences under
 ``Account → Preferences``. By default, the Core only provides some
@@ -77,14 +77,14 @@ apps.
       sent notifications to you (cf. [@sec:Notifications]).
 
 Third-party accounts
-~~~~~~~~~~~~~~~~~~~~
+--------------------
 
 If you logged in using a third-party account (e. g. a Google or
 Microsoft account), you can manage the connections to these accounts on
 the page ``Account → Third-party accounts``.
 
 Authorized applications
-~~~~~~~~~~~~~~~~~~~~~~~
+-----------------------
 
 On the page ``Account → Authorized applications`` you can see all
 external applications you authorized to retreive data about you from