Add links to release headers (#10808)

Each release header now includes a link to the range of commits
included with that release. These links are at the end of the document,
in accordance with the "keep a changelog" [1] format.

For the purpose of this changelog, the "previous release" is the most
recent release mentioned in the changelogs. The diffs ignore any
releases that were omitted from the changelog. This is mainly an issue
with older releases, so it seemed acceptable. All releases have been
documented for a couple of years now, and will be going forward as
well.

The name of the "Current Develop Branch" section was changed to
"Unreleased" to confirm with "keep a changelog".

The `auto-changelog.js` script has been updated to update/add these
links whenever adding a new release header as well.

[1]: https://keepachangelog.com/en/1.0.0/