Adding version support to docs

[shimwell]

Description

This PR updates the documentation creating CI so that it builds docs with the Pydata theme. Docs are built every PR and push as previously and this now gets put in the dev folder on the gh-pages branch Docs are now built with every tag release and this gets put in a folder with the version number on the gh-pages branch

I've investigated 5 ways of adding versions to sphinx based docs and am convinced that the pydata is the best option currently and used by famous packages like numpy

• https://github.com/Smile-SA/sphinx-versions no longer maintained does not work with latest sphinx
• https://github.com/Holzhaus/sphinx-multiversion no longer maintained does not work with latest sphinx
• https://www.codingwiththomas.com/blog/my-sphinx-best-practice-for-a-multiversion-documentation-in-different-languages nice approach, does not need extra packages but needs the most coding adding to geouned repo
• https://pydata-sphinx-theme.readthedocs.io/en/stable/ looks like the best option
• https://about.readthedocs.com (requires another login and admin accounts, includes adverts)

Happy to go into more details but in summary the pydata-sphinx-theme does everything required

I will need to follow up this PR with some changes directly to the GH-pages branch before everything works nicely

In the meantime here is a version I made on my fork https://fusion-neutronics.github.io/GEOUNED/

Fixes issue

Please link to any issues that this PR fixes

Checklist

• [x] I'm making a PR from a feature branch on my fork into GEOUNED-org/GEOUNED/dev branch
• [x] I have followed PEP8 style guide for Python or run a formatter such as black or ruff format on my code
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works

[shimwell]

This one is ready for review if anyone has time