docs versions + updates

[casperdcl]

Provide multiple (not just latest) docs versions

• use/fix https://cil.readthedocs.io docs: redirect readthedocs to github pages #1693
• drop/update GH conda docs builds docs: native build, update sphinx, add versioning #1679
  • redirect https://tomographicimaging.github.io/CIL/{,nightly} -> https://cil.readthedocs.io
  • closes Speed up docs action by using build environment from conda build action #1430
  • closes Update to use v3 of upload-artifact in github-actions #1380
  • fix/update sphinx (closes update sphinx #1597)
• multiple versions docs: native build, update sphinx, add versioning #1679
  • latest/nightly/master, vM.m.p
  • fix version switcher .html suffix docs: fix version switcher #1797
  • delete fork after upstream release fix version-switcher with dirhtml builder pydata/pydata-sphinx-theme#1795 i.e. pydata-sphinx-theme>=0.15.3
• meta-docs docs: explain docs folder layout #1830
• developer vs user docs (e.g. unhide _private methods)
• CIL-Demos ipynb rendering Doc render jupyter #1690
• add UML UML diagrams #1688
• link to GitHub source code rather than render source code statically
• create a nicer landing page docs: add jekyll landing page #1699
  • improve widgets website: improve widgets #1815
  • content docs: update landing, about, & footer #1831
    • https://ccpi.ac.uk/cil/
    • Tomography-RALTeam/Website content

[gfardell]

I think we want 'https://tomographicimaging.github.io/CIL' to be the CIL entry point. Currently that's probably the github CIL readme.md, but we also have a page on https://ccpi.ac.uk/cil/ . This would mean having pages introducing CIL, installation instructions, code documentation (should we have user and developer versions too?), tutorials and notebooks, and a contribution guideline. Similar to this project from @jakobsj https://cuqi-dtu.github.io/CUQIpy/

We had a lot of feedback last week on having our information split across multiple sites so I'd be nervous about bringing readthedocs in to it too. I think this is something that will be discussed at vthe next developer meeting.

[casperdcl]

The thing is readthedocs gives us versioning for free. If we want that then it's better to migrate away from gh-pages... #1673