Automatically document API endpoints

This should be similar to the Sphinx Autodoc extension, so users add something like `.. apidoc: mapper.*` to the docs at an appropriate location, and every matching API method is automatically documented.

This will also need to support some kind of documentation of REST resource types and their keys, where such things exist.

 * [x] Render docstrings to HTML
 * [x] Render docstrings via an API call
 * [x] handle param descriptions, :returns:
 * [x] Filter API calls by blueprint
 * [x] Index API calls, with a way to refer to them from elsewhere
 * [x] Resource type documentation in the source code
 * [x] Index resource types, refer to them from elsewhere