[Docs] Document how to create psoter groups

docs/admin/00_index.rst

+Providing time-based documents with Resint
+==========================================
+
+.. toctree::
+   :glob:
+
+   *

docs/admin/01_poster_groups.rst

+Uploadable posters for time-based documents
+===========================================
+
+Posters are documents that can be manually supplied in a
+time-based manner. Example use cases are cantine menus,
+that are provided weekly by an external supplier, who
+will only get privileges to upload this poster as a PDF
+file.
+
+Poster fiels can be uploaded for a defined time period,
+and AlekSIS will then deliver the currently valid version
+under a stable menu item and URL.
+
+Defining poster groups
+----------------------
+
+Uploadable posters are categorised into poster groups, with
+each poster group representing one time-based document that
+AlekSIS will provide. Each poster group has:
+
+ * A name
+ * A URL slug making up the final, stable URL
+ * A schedule defining the cycle for providing the document
+ * A default PDF file for periods for which no document is uploaded
+
+Permissions to upload documents are managed per poster group
+as well.
+
+Creating poster groups
+~~~~~~~~~~~~~~~~~~~~~~
+
+Poster groups are created from the menu under the `Documents → Poster groups`
+menu item. After clicking the `Create new poster group` button or the `Edit`
+action for any poster group, the following form allows editing the details
+of the poster group.
+
+.. image:: ../_static/create_poster_group.png
+  :width: 600
+  :alt: Create or edit a poster group
+
+In addition to the properties explained above, the form provides two options
+to configure if and how the document will be presented to users. It is possible
+to include or exclude the document from the main menu, and to allow access
+by anonymous users. Documents that are hidden from the menu can be used if
+they should only be accessible through links from external sites, or
+loaded by a digital signage system.

docs/user/00_index.rst

-Using time-based documents
-==========================
+Publishing time-based documents with Resint
+===========================================
 
 .. toctree::
    :glob: