Automate release process

[laymonage]

Noticed a few things when I was releasing v6.3.0 just now:

• Our git tags are inconsistent: some start with v, some don't: https://github.com/wagtail/sphinx-wagtail-theme/tags
• In the release process guide, there are prerelease and postrelease commands that I don't know where they come from. They do not seem to be necessary.
• The python -m build command doesn't seem correct. It should be python setup.py sdist bdist_wheel or make build.
• The requirements.txt does not include wheel and twine, so these had to be installed separately. Either include them in the requirements.txt, or call this out explicitly.

That said, it would be nice to automate the release process using GitHub Actions. We already build the project in our CI config, we can add additional step that publishes the package using the official PyPI action and the Trusted Publisher configuration. I use this for modeladmin and it's working great: https://github.com/wagtail-nest/wagtail-modeladmin/blob/main/.github/workflows/publish.yml.

cc @Stormheg

[lb-]

I agree about the pre/postrelease - I have no idea where that comes from. It worked on my old laptop but not on my new one and in the last release I just manually created the changes.