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::.

Built the docs and went through them. Verified the links worked and the

code segments were all highlighted.
Didn't see any new errors or warnings during docs generation.