[Docs] Add user preferences

docs/user/40_preferences.rst

+Configuring the timetable display
+=================================
+
+Some display options can be changed by the user, under `Account → Settings → Timetables`:
+
+ * **Shorten groups in timetable views**: With this setting activated, lists
+   with more groups than set in the system-wide limit will be collapsed into
+   a shorter string (e.g. 5a,b,c,d,e,f might be abbreviated as 5a-f)