Use GitHub pages direct deployment

[coretl]

At the moment we push to the gh-pages branch. This adds to the size of the repo, and this is why we switched to only publishing main and releases. We should try using the new pages deployment like DiamondLightSource/python3-pip-skeleton#133. The downside is that we cannot incrementally deploy, we need to push the whole site each time. To do this we should:

• Always build docs, and store as artifact
• Maintain switcher.json manually
• On tag then store artifact on release and create a PR to update switcher,json
• On push to main, create site from main + unzipped artifacts from all releases in switcher.json
• Make a migration that uploads every tag directory in gh-pages to the relevant release

[coretl]

If we wanted to maintain branch builds as well we could grab from any valid artifact caches, but we have the problem of what happens if you push a branch build and the main build artifact cache has expired.

The other alternative to this is to keep using the gh-pages branch, but squash on each deploy. The action almost does this, but not quite until peaceiris/actions-gh-pages#455 is fixed