Add release notes for Review Board 3.0.

Review Request #9393 — Created Nov. 19, 2017 and submitted — Latest diff uploaded


Review Board


It's finally here. This change adds release notes for Review Board 3.0,
covering integrations, status updates, avatars, general comments,
Ship-It revocation, OAuth2, Emoji, and all the other major changes we've
spent a small eternity working on.

Most of the major features have screenshots showing off the
functionality, helping provide a visual tour of the release. The major
features also have their own sections, with smaller feature improvements
bundled together under more general sections.

Many of the screenshots used both here and in earlier 3.0 beta release
notes have been updated to fix some visual consistency issues and to
reflect the evolution of some of the features.

Anchors have also been added to several doc pages, allowing us to
reference them with "Learn more" links for many of the features.

Built the docs. Checked for linking errors, build errors, spelling
errors, and visual inconsistencies in the screenshots.