This updates our docs to reference the Review Board dev docset (which

we'll want to bump to 4.0 later), and to remove some old, unused

intersphinx mappings and add more future-proof ones.
The Review Board doc version is now defined as variables in

djblets/dependencies.py, for easier maintenance.
A syntax error was also fixed in a docstring.

Built the docs and release notes. Checked references and checked

for build errors.