Update documentation references and syntax errors.
Review Request #11285 — Created Nov. 16, 2020 and submitted
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.