Improve & update release process to reflect recent changes (#3242)

- Formalise release cadence guidelines
- Overhaul release steps to be easier to follow and more thorough
- Reorder changelog template to something more sensible
- Update release automation docs to reflect recent improvements (notably
  the addition of in-repo mypyc wheel builds)

Co-authored-by: Felix Hildén <felix.hilden@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>