Allow to specify defaults for filters via URL params
This allows you to specify defaults for filter parameters using URL parameters.
It works, for example for the "Contact in Group filter". Use the name of the filter on the URL eg. &group_id=1.
I also added documentation which explains better.
I also added a basic mkdocs.yml (but does not yet have all docs pages) so it can be added to docs.civicrm.org in the future.