docs: fix version switcher

[casperdcl]

• fix version switcher suffixing .html
  • i.e. we want /CIL/nightly/ not /CIL/nightly.html
  • vis. fix version-switcher with dirhtml builder pydata/pydata-sphinx-theme#1795
• consistently sort version switcher dropdown options
• misc framework updates
  • push docs on tagged release
  • suppress missing html_static_path build warning

Testing you performed

https://github.com/TomographicImaging/CIL/blob/fix-docs-version-switcher/.github/workflows/README.md#docs

Related issues/links

• docs versions + updates #1579
  • fix version-switcher with dirhtml builder pydata/pydata-sphinx-theme#1795
  • docs: add jekyll landing page #1699

Checklist

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have updated the relevant documentation
• 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

[gfardell]

How is it best to review this? Could you either add a screenshot to show functionality or maybe demo it in the stand-up?