docs: native build, update sphinx, add versioning

[casperdcl]

Describe your changes

• native CI build (fixes Update to use v3 of upload-artifact in github-actions #1380)
  • ⚡ <3 mins to builds CIL & render docs
• unpin docs deps & remove inconsistencies/duplicates (fixes update sphinx #1597)
  • 🏷️ manually created versions.json as per the spec at https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/version-dropdown.html#add-a-json-file-to-define-your-switcher-s-versions
• misc style improvements
  • 😎 auto, dark, light themes, site logo & favicon
  • remove right sidebar
  • replace Show Source with 📝 Edit on GitHub link

Notes

• managed to stick to tomographicimaging.github.io - no need for readthedocs :)
• version dropdown list includes alias for stable -> latest tag
  • ❓ do we also want tomographicimaging.github.io/CIL/stable to work?
  • 😖 did not use https://github.com/Holzhaus/sphinx-multiversion as it only handles docs not code (i.e. "old" docs versions will link to latest master code)
  • 😖 did not use https://github.com/sphinx-contrib/sphinxcontrib-versioning nor https://github.com/Smile-SA/sphinx-versions because they are borked
• follow-up: other items in docs versions + updates #1579
• follow-up: run tests as part of docs job (part of CI matrices #1598)

github ref	destination
master	https://tomographicimaging.github.io/CIL/nightly
vM.m.p	https://tomographicimaging.github.io/CIL/vM.m.p
#123	DocumentationHTML:/123_merge

• https://tomographicimaging.github.io/CIL/ redirects to https://tomographicimaging.github.io/CIL/nightly

follow-up (not that trivial to do):

github ref	destination
latest vM.m.p	https://tomographicimaging.github.io/CIL/stable

• https://tomographicimaging.github.io/CIL/ redirects to https://tomographicimaging.github.io/CIL/stable

Describe any testing you have performed

• Download & unzip the DocumentationHTML art[ie]fact from the GitHub Actions run
• run python -m http.server

Link relevant issues

• part of docs versions + updates #1579
• related Speed up docs action by using build environment from conda build action #1430

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License.
• I confirm that the contribution does not violate any intellectual property rights of third parties

[lauramurgatroyd]

I tested the new docs_environment.yml locally and it worked well!