Remove versions from sidebar

[mccalluc]

• Fix Can the release list sidebar sort v.10 to the top? Or maybe remove version list from sidebar? #1712
• We have enough versions that this is becoming distracting, and we don't want to steer users to older versions in any case.
• Ok to keep the branches section?
• Quick scan of other projects show none with versions in sidebar, and instead they have a menubar button; I don't think it's essential, but we could add it as a replacement.
  • https://pydata-sphinx-theme.readthedocs.io/en/stable/
  • https://docs.pola.rs/api/python/stable/reference/index.html
  • https://scikit-learn.org/stable/api/index.html
  • https://pandas.pydata.org/docs/reference/index.html

[mccalluc]

• Close in favor of Add warning on docs for older versions #1986
• The "right" way to do this with the pydata theme requires a new JSON file with the version info: Not worth figuring that out for the negligible gain.