[Docs] Add notes about substitution plan

docs/user/10_substitution_plan.rst

+Substitution plan
+=================
+
+In addition to the `smart plan`, substitutions can be
+displayed as a traditional substitution plan. The substitution
+plan lists all changes made to the regular plan, on a
+daily basis, as a table.
+
+View in AlekSIS
+~~~~~~~~~~~~~~~
+
+In AlekSIS, the substitution plan is available from the menu under
+`Timetable → Substitutions`. This function provides a browsable list,
+where substitution plans can be navigated by week.
+
+Print view
+~~~~~~~~~~
+
+Like for the regular timetable, the substitution plan can be
+printed (as a PDF file), to make hardcopies, or to display it
+on digital signage.