Our extensions framework actually had pretty decent documentation, but

it was quite poorly organized.
This change revamps things, primarily the main "Extending Review Board"

page.
That page used to just be a set of links to detailed reference pages and

a huge list of individual hooks. It's now much more of a tutorial, going

through the basic steps of setting up a development environment,

creating the extension boilerplate (via rbext create), installing that

package in editable mode, activating it, and then the basics of hooks

and API resources.
The list of full examples is now more prominent, coming immediately

after those initial steps.
The "Extension Basics" section has been renamed to "Extension

Reference", and the toctree has been given one more level, letting

people see what's inside each of those pages (since some of those have a

lot of specific detail which may not be immediately obvious from the

title).
The list of hooks has been moved into its own page and reorganized to

hopefully be a bit clearer. The descriptions for each hook are generally

simplified but also expanded a bit in cases where it's a bit more niche

(such as approval and diff ACLs).

Built docs and went carefully through all the pages.