The Review Board release notes have been built up over a long period of
time, and during that time we've introduced codebase references, new
documentation, more complex release notes, and as a result, lots of
improper references.

These references result in Review Board 1.7/2.0 release notes linking to
2.5 documentation/code references. Going forward, they would end up
linking to 3.0 documentation, then 4.0, etc.

This change introduces a much-needed cleanup of the release notes. We
now have intersphinx mappings set up for each major Review Board,
Djblets, and RBTools release. We then explicitly use the intersphinx
prefix for the appropriate docs, ensuring that each release notes doc
links to the docs for the correct version of Review Board.

There are also small fixes throughout, such as resolutions between
conflicting anchor names and small syntax error fixes.

Built the docs. Verified that there weren't any warnings or errors,

and that links in the docs were linking to the proper versions.