Use official actions to publish the API docs

[twm]

We can now publish pages directly from actions so I composed two official actions:

• actions/upload-pages-artifact
• actions/deploy-pages

I also:

• Switched the Pages source to GitHub Actions
• Ticked the "Enforce HTTPS" box
• Removed the environment protection rule that allowed deploys from the gh-pages branch
• Unticked the "allow admins to bypass" box, since it appears to have allowed a deploy from this PR to go live (!)

Fixes #98.

[adiroiban]

Thanks. It looks good.

Since the GitHub Pages preview functionality is not yet available, we can't deploy the docs for a PR, like we can on Read The Docs.

I guess that the only option is to merge and check that it worked on trunk.

[twm]

Thank you again @adiroiban!