[Docs] Add information about setting up notifications in admin handbook

docs/admin/11_notifications.rst

+Setup notifications about current changes
+=========================================
+
+Users can get notifications about current changes to their personal timetables.
+To activate this behavior, the system administrator has to ensure multiple things:
+
+* The data come from a compatible source, for example, AlekSIS-App-Untis.
+* The notifications have been activated in the preferences (see below).
+* There is at least one notification channel available to your users (cf. :ref:`core-admin-notifications`).
+
+Preferences
+-----------
+
+You can customize the way how and when notifications are sent at the configuration page at **Admin → Configuration → Timetables**:
+
+* **Send notifications for current timetable changes:** With this checkbox, the whole feature can be activated or deactivated.
+* **How many days in advance users should be notified about timetable changes?** Here the number of days can be configured notifications will be sent 
+  before the actual affected day. A common value is one or two days.
+* **Time for sending notifications about timetable changes:** At this time, the notifications for the next days will be sent. 
+  This is only used if the changes are created before the period configured with the above mentioned option. If they affect a day in this period,
+  the notification will be sent immediately.