[Docs] Document live documents

docs/admin/02_live_documents.rst

+Configuring live documents
+==========================
+
+Live documents are another type of time-based documents.
+In contrast to posters, they are not uplaoded manually,
+but automatically updated by some trigger.
+
+Types of live documents are provided by other AlekSIS apps.
+All live document types provided by apps can then be configured
+from the `Documents` menu.
+
+An example for a live document is an automatically generated
+substitution plan, which can be linked to from a website,
+or displayed on digital signage.
+
+.. image:: ../_static/create_live_document.png
+  :width: 600
+  :alt: Create live document
+
+All fields except the slug are provided by the app.
+The slug again makes up the stable URL the live document
+will be provided under, which can be copied from the live document
+list.
+
+The app providing a live document type will take care of
+updating the document. A live substitution plan might thus be
+re-generated whenever timetable or substitution data changes.