[Docs] Add sync docs

docs/admin/20_syncing.rst

+Synchronizing with Matrix
+=========================
+
+To initially start the synchronization of groups in AlekSIS to rooms,
+and optionally spaces, to Matrix, go to ``Matrix → Groups and Rooms``.
+
+Select the groups to be synchronized and click ``Execute``.
+This process can take some time.
+All group members that have a Matrix profile in AlekSIS, that may be automatically
+created when a ``Name of Matrix homeserver user for auto-generating Matrix IDs`` is set,
+get invited to the respective rooms.
+
+After this initial synchronization, every change in a ``Group``, ``Matrix room`` or
+``Matrix profile`` will be refleceted in Matrix.