Update the documentation on installing development versions.
Review Request #11472 — Created Feb. 15, 2021 and submitted
We have a very old set of docs on how to install development versions of
Review Board. This was out-of-date, and though it wasn't directly linked
to in the main index pages of the manual, it was still available.
This change updates these docs to be more future-proof and useful to
link to, and directs users to the release notes of a particular release
for more specific instructions.
Viewed the docs. Tested links, build output, and spelling.