Perform docs builds from this repo

[jamietanna]

As noted in #39604, we've recently migrated the Renovate docs repo to update the Renovate version (in the docs site) via a tag, instead of via a Git submodule. This means that docs changes are not landing until a release is made, whereas previously, it was possible to have a submodule commit updated to include a commit merged on main that hadn't yet been released.

Given we no longer need to have a separate repo, we can instead replace the separate docs repo with this repo instead publishing.

(See also: renovatebot/renovatebot.github.io#108)

This would cover:

• Amend CI workflow to:
  • Add any new script(s)/file(s) needed from the docs repo
  • Build docs site on main
  • Or if a ci:docs label is added
  • Publish built docs (if they are as part of CI) to GitHub Actions Artifact
  • Publish docs builds on main to GitHub Pages
• Once GitHub Pages are verified as correctly publishing, migrate CNAME from one repo to the other
• Update internal documentation about updates to the docs site
• Archiving the docs repo

Outstanding decisions:

• Should we migrate back to "every commit on main pushes the docs site"