Add release notes and bump the version for Djblets 2.3.
Review Request #11991 — Created Jan. 24, 2022 and submitted — Latest diff uploaded
This is a bug fix, compatibility update, and small feature release,
bringing better compatibility with Python 3.10, Python-Markdown 3.x,
and preparing for newer versions of Django.
Built the docs. Checked for bad links, spelling, and build errors.