[Docs] Document user invitation and registration

docs/admin/22_registration.rst

+Registration and user invitations
+=================================
+
+Configuration
+-------------
+
+.. _core-registration:
+
+Registration
+~~~~~~~~~~~~
+
+Registration can be enabled via the configuration interface in frontend.
+
+In the ``Authentication`` tab, click the checkbox ``Enable signup`` to enable
+signup for everyone.
+
+User invitations
+~~~~~~~~~~~~~~~~
+
+.. _core-user-invitations:
+
+User invitation can be enabled and configured via the configuration
+interface in frontend.
+
+In the ``Authentication`` tab, click the checkbox ``Enable invitations``, to
+enable invitations.
+
+You can also configure the length of the invite code, defaults to ``3``, and
+the size of packets, defaults to ``5``.
+
+By default, an invitation code looks like the following:
+``abcde-abcde-abcde``
+
+Usage
+-----
+
+Invite by email or code
+~~~~~~~~~~~~~~~~~~~~~~~
+
+To invite a new user , visit the invitation page located at ``People → Invite
+person``
+
+Here you are able to invite the user by email address or generate an
+invitation code.
+
+.. image:: ../_static/invitations.png
+  :width: 100%
+  :alt: Invitations page
+
+Invite existing person
+~~~~~~~~~~~~~~~~~~~~~~
+
+To invite an existing person, open the Person in AlekSIS an click ``Invite
+user``.
+
+The invitation will be send to the persons email address.
+
+.. image:: ../_static/invite_existing.png
+  :width: 100%
+  :alt: Invite existing person

docs/user/01_registration.rst

+Register a new account
+====================
+
+.. _core-user-registration:
+
+Public registration
+-------------------
+
+If public registration is enabled on the AlekSIS instance, you can click the
+``Signup`` button located in the navigation sidebar and register for an
+AlekSIS account.
+
+.. image:: ../_static/signup.png
+  :width: 100%
+  :alt: Signup formular
+
+Verify email address
+~~~~~~~~~~~~~~~~~~~~
+
+If enabled, you have to verify your email address after signup.
+
+To do so, click on the link you recieved on the email address you entered in
+the signup form.
+
+Using an invitation code
+------------------------
+
+If you have an invitation code, click ``Accept invitation`` in the sidebar and
+enter it. You will be redirected to the signup formular.
+
+.. image:: ../_static/accept_invite.png
+  :width: 100%
+  :alt: Accept invitation
+
+Using an invitation link
+------------------------
+
+If you've recieved an invitation link (e.g. via email), clicking the link
+will redirect you to the signup form.