Improved release automation

This is a tracking issue for the various release automation improvements were are planning to make.

- [ ] Tag & publish tip of release branch, rather than after release branch merge.
* This will let us drop the `master` branch, and target the main branch instead (`develop`)
* Any conflicts between the release branch and the main branch should be resolved *after* publishing, but *before* merging the release branch. This means that the trigger for starting the release process cannot be the merging of the release branch. It'll have to be triggered by something else (e.g. a GitHub comment).
- [x] Run changelog generation script automatically when preparing an RC
- [x] Improve automatic changelog generation
* It should generate a changelog in the ["keep a changelog" format](https://keepachangelog.com/en/1.0.0/), which each commit initially uncategorized.
* it should exclude any entries already listed in the changelog
* It should group new entries under the header of the release currently being prepared
* The release date should be omitted for release candidates.
- [ ] Update release changelog entry with release date when the GitHub release is created
- [ ] Write instructions for new release process
* These should be included in the repository
* These instructions should assume as little as possible. Include examples of each command that should be run.
* Include instructions for both normal and hotfix releases
* Include instructions on how to monitor for regressions after the release has been published