Use GitHub pages direct deployment

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 https://github.com/DiamondLightSource/python3-pip-skeleton/pull/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