API - sorting and notes

[geoff-maddock]

Fix issues with sorting API endpoints and update the docs.

• Make sure we can sort by relation properties, either all, a subset, or an aliased list of options, for instance venue.name
• Note how to do so in the API docs.
• Make sure the main docs links to the API docs correctly

To order by a specific field, use the field name as the key and the value as the value.
Example: GET /api/events?sort=model.property&direction=asc