Split up the extension docs.

The extension documentation was getting a bit too large, making it hard
to find specific content. This splits the docs into a number of new
pages:

* Overview
* File Layout
* Extension Class
* Extension Configuration
* Database Models
* Extension Hooks
* Static Files
* Review UI Integration
* Extending the Web API
* Extension Distribution

Otherwise, the content is largely unchanged, with the exception of some
cross-links and converting implicit code blocks to using
`..  code-block::`.