Set up CI

[ElliottKasoar]

• Testing (e.g. https://github.com/stfc/janus-core/blob/main/.github/workflows/ci.yml)
  • Run unit tests, ideally against all supported versions of Python
  • Check the documentation can be built
  • Check pre-commit passes
  • Runs on all pushes/PRs
• Build and publish documentation
  • Probably runs on commits to main
• Build and publish package to PyPI
  • Probably runs tagging of versions