Document the Plugin API

[alexandratran]

Add a manually formatted reference of the Plugin API. The existing Javadoc spec might be used as a starting point, but should be updated to reflect the current API.

This section should document each API interface, method, etc., with a description, parameters, return values, and an example usage. It might be broken up into separate pages in this category, e.g., one page for each interface.

This topic should go under Plugins > Reference (see #1560).