docs versions + updates

[casperdcl]

Provide multiple (not just latest) docs versions

• [x] use/fix https://cil.readthedocs.io #1693
• [x] drop/update GH conda docs builds #1679
  • ~redirect https://tomographicimaging.github.io/CIL/{,nightly} -> https://cil.readthedocs.io~
  • closes #1430
  • closes #1380
  • fix/update sphinx (closes #1597)
• [x] multiple versions #1679
  • [x] latest/nightly/master, vM.m.p
  • [x] fix version switcher .html suffix #1797
  • [ ] delete fork after upstream release https://github.com/pydata/pydata-sphinx-theme/pull/1795 i.e. pydata-sphinx-theme>=0.15.3
• [ ] meta-docs #1830
• [ ] developer vs user docs (e.g. unhide _private methods)
• [x] CIL-Demos ipynb rendering #1690
• [ ] add UML #1688
• [ ] link to GitHub source code rather than render source code statically
• [x] create a nicer landing page #1699
  • [x] improve widgets #1815
  • [ ] content #1831
    • [ ] https://ccpi.ac.uk/cil/
    • [ ] Tomography-RALTeam/Website content