Add release notes for Review Board 6.

Review Request #14 — Created Oct. 22, 2023 and updated


Review Board


This is a large release, introducing the Unified Review Banner, look and
feel improvements, better diff navigation, a Markdown formatting
toolbar, visuals for file attachment state, self-review of draft diffs,
extension improvements, service health checks, and numerous other
improvements and fixes.

The release notes cover all this and provide some screenshots. An
upcoming change will improve upon the screenshots in the release notes,
as new multi-DPI screenshots are added to the documentation.

Testing Done:
Built the docs. Checked for build errors, spelling errors, and bad

Tested the screenshots. There's work that needs to happen in the
rbwebsite code to fix clicking screenshots (important for the larger
ones), and more screenshots are needed, but those will come later.

Reviewed at