This new helper script takes a branch name as input, then fetches
`CHANGELOG.md` from that branch in the upstream mixxxdj/mixxx repo,
converts the changelog into sphinx's ReStructured Text format and writes
it to `source/chapters/appendix/version_history.py`.

This makes keeping the manual repository changelog in sync with the code
repository changelog much easier and will allow automation in the
future.

This workflow can be triggered using a personal access token (PAT) with
the `public_repo` scope.

    $ curl -H "Accept: application/vnd.github.v3+json" \
           -H "Authorization: token <your-token>" \
           --request POST \
           --data '{"event_type": "update-changelog", "client_payload": {"branch": "2.3"}}' \
           https://api.github.com/repos/mixxxdj/manual/dispatches

It will then run `tools/update_changelog.py` and if any files were
changed, it will commit these changes and then trigger rebuild of the
manual.

For the latter to work, a `MIXXXBOT_CHANGELOG_AUTOUPDATER_PAT`
repository secret needs to be present, because the default
`GITHUB_TOKEN` doesn't suffice.

This is now part of mixxxdj/mixxx#4256.

As requested here: mixxxdj#422 (comment)