Add docs builds to CI

[ajjackson]

Closes #34

For now let's have a strategy that

• Docs are built on every push to PR
• They are saved as an Artifact if the docs action is called by Workflow Dispatch
• The release process includes a workflow dispatch or call to build docs and publish to Github Pages
• Once the release pipeline is in place, critical docs improvements should be published with a post-release to the main code

[ajjackson]

I have intentionally escalated warnings to errors to see if that makes a red cross. It does!

[ajjackson]

Succesfully deployed to https://pace-neutrons.github.io/resins/ 🎉

(Will now remove this branch from the protection rule so that only main can deploy to pages.)