Use towncrier for release notes

[furious-luke]

Towncrier is a neat little project that assists in automatically preparing release notes for projects. I think it could be useful to automate this process. A couple things that we should consider:

• There are precommit hooks and checks we can run to ensure PRs come with release notes.
• We can add the release note collection stage to the CI pipeline.

[banagale]

This looks like it could be really useful. It would be better to assemble release notes as PRs are pulled in than to add weight to the end.