Fix CI status by moving docs deployment to GHA

[janbrasna]

Currently the CircleCI secrets only have read-only access scope for the GitHub token, being unable to write the built docs to gh-pages branch. It's the only failure in the pipeline, and instead of providing write permissions it may be easier to just move the actual docs deployment to GitHub Actions, avoiding any commits to the pages' branch and publishing straight to the deployment env without any underlying git tree being written etc.

This PR keeps the docs-build step in CI for consistency in testing, only removing the actual docs-publish and creating a workflow here instead.

Note

To enable Pages publishing via Actions, you may need to to change the publishing source in repo settings for this workflow to take over, and also make sure if there are any env protections defined they also include "master" and not "gh-pages" only.)

Once you're happy with the trigger-based deploys from master, you can safely remove the gh-pages branch completely (which hasn't received updates for more than two years) as it won't be used anymore, and also remove the tschaub/gh-pages yarn dev dependency and relevant scripts as all docs will be published from master branch deploys automatically from that point on.

[janbrasna]

You can check the sample run for b05d1a6 ^^ logged from the fork.