Add customisation help to docs (#984)

* Make sure user-defined themes folders exist in the data path
* Minor cleanup in shortcuts table
* Add details in docs on how to customise GUI and add dictionaries
* Add spell check reference in docs overview

docs/source/int_overview.rst

@@ -21,6 +21,8 @@ applications.
 
 For install instructions, see :ref:`a_started`.
 
+For information on how to add spell check dictionaries, see :ref:`a_custom_dict`.
+
 Using novelWriter
 =================