The old Extending Review Board page was just a big Table of Contents,

which wasn't very useful to users who were just getting started. This

change begins the first steps toward improving our documentation for

this section of the manual by redoing the main page to better guide

users through the various extension tasks, lists of hooks, and by

pointing to useful code references.
There are also some changes to a few of the other smaller pages that

were previously linked to. Some pages are no longer necessary and now

link back to the main index page (so as to not break links/search

results). One page (signal-hook.rst) has a couple small updates to link

better to Django documentation.
Upcoming changes will flesh out the content further.

Built the docs and read through the front page. Tested the different

links and checked for broken links.