Minimal automated release process

[jdangerx]

With #3140 and #3195 addressed we're very close to being able to make production software + data releases automatically after pushing a release tag (anything starting with v20*). What remains to be done?

Instruction manual will include the following steps:

• close out release the night before
• tell people you're planning on releasing
• push tag, make sure it points at nightly-20whatever
• verify: RTD, distribution buckets, Zenodo (update metadata, see if the data all makes sense, etc.)

Scope

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1. Modify the nightly build script to create a draft data release on the production Zenodo server when doing a v20XX release.
2. modify release.yml so that test-pypi release always happens before pypi release
3. Add a release instruction manual
4. Add documentation pointing people at stable branch for stable releases.