This is instead of pushing directly. Creating a pull request is slightly
more work for the release manager, but it is more secure as we no longer
need a secret with write access to the www-releases repo.
We currently receive a warning on all Github Actions workflows that use
`setup-python`, since they all use v4 of the action, which uses the
deprecated Node.js 16. This PR upgrades the action in all places to v5,
which uses Node.js 20 (see [setup-python release
page](https://github.com/actions/setup-python/releases/tag/v5.0.0))
* Split out the lit release job and the documentation build job into
their own workflow files. This makes it possible to manually run these
jobs via workflow_dispatch.
* Improve tag/user validation and ensure it gets run for each release
task.