documentation: area coordinator capabilities

[redshiftzero]

We have a non-admin role on the web application that can:

1. Add or remove officers.
2. Add notes to officer profiles.
3. Add incidents, links, or videos to officer profiles.
4. Add new ranks for officers.
5. Maybe other stuff that I'm forgetting 🙃

The implementor of this ticket should go through the webapp, enumerate any other capabilities of the area coordinator, and write a page of documentation targeted at new area coordinators.

See the documentation quickstart here on how to edit the docs.