docs: add release process documentation

[maresb]

Summary

• Add RELEASING.md documenting the release process
• Codifies the existing ad-hoc practice into a consistent checklist: version bump, changelog generation, PR, GitHub Release

Made with Cursor

[maresb]

@aseyboldt, I generated this in the process of releasing v0.16.8. Not sure if it's worth an additional file in the project root, but I think it'd save me substantial time for the next time I (or potentially someone else) cut a release.

[aseyboldt]

Thanks, looks good!