Update docs for 4.0

docs/_static/all_timetables.png

docs/_static/change_lesson.png

docs/_static/class_timetable.png

docs/_static/daily_lessons.png

docs/_static/group_timetable.png

docs/_static/my_timetable.png

docs/_static/plan_substitutions.png

docs/_static/plan_substitutions_select_teacher.png

docs/_static/print_timetable.png

docs/_static/substitutions.png

docs/_static/substitutions_print.png

docs/_static/timetable_calendar.png

docs/_static/timetable_calendar_details.png

docs/admin/10_managing_data.rst

docs/admin/11_notifications.rst

docs/admin/40_preferences.rst

docs/user/01_using_timetables.rst

docs/user/10_substitution_plan.rst

docs/user/11_notifications.rst

docs/user/15_manage_substitutions.rst