[Docs] Add information on how to use notifications for users

docs/user/11_notifications.rst

+Notifications about current changes
+===================================
+
+Users can get notifications about current changes to their personal timetables.
+If the system administrator has enabled the feature, they can activate it by
+setting the personal preference **Send notifications for current timetable changes** (cf. :ref:`core-user-preferences`).
+
+The notifications are sent as standard AlekSIS notifications. In :ref:`core-notifications`, you can find more information
+about the available channels and your configuration options.
+