Add release notes for Review Board 4.0.5.
Review Request #11998 — Created Jan. 24, 2022 and submitted
This is a sizable architectural, feature, and bug fix release,
featuring new diff parsing capabilities, experimental DiffX and diff ACL
support, compatibility fixes for MySQL, upgrade fixes, new integration
capabilities, and more.
Built the docs. Checked for build errors, spelling errors, and bad links.