Add release notes for Review Board 6.
Review Request #14 — Created Oct. 22, 2023 and updated
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 links. 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 https://reviews.reviewboard.org/r/13342/