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

Review the current gh-pages deployment and the referenced GitHub Pages deployment PR first. Trace how documentation artifacts, switcher.json, tags, releases, and main are handled. Done means docs are built as artifacts, release and main deployments assemble the required site, and existing gh-pages tag directories are migrated to releases.

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