This adds the release notes for the new work going into Djblets 3.0

beta 2. This release focuses primarily on new API token generators and

extension loading fixes.
Missing docs for the new token generators have been added to the Djblets

code reference guide, allowing the release notes to link properly, and

some bad roles have been fixed in the 3.0 beta 1 release notes.
The branch information used for source links have also been updated to

use our current branch naming scheme.

Built the docs and the release notes. Verified that all links work and

the release notes didn't encounter build errors.
Checked for spelling errors.