[Docs] Add tons of screenshots

docs/_static/assign_group_role.png

docs/_static/create_excuse_type.png

docs/_static/create_extra_mark.png

docs/_static/edit_group_role.png

docs/_static/excuse_types.png

docs/_static/extra_marks.png

docs/_static/group_roles.png

docs/_static/lesson_documentation.png

docs/_static/lesson_personal_notes.png

docs/_static/lesson_version_history.png

docs/_static/my_groups.png

docs/_static/my_students.png

docs/_static/overview_lessons.png

docs/_static/overview_person.png

docs/_static/register_absence.png

docs/_static/register_absence_confirm.png

docs/_static/students_list.png

docs/_static/week_view.png

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docs/admin/30_configure.rst