Remove versions from sidebar in docs

[dstansby]

My reasoning here is:

• There is now a version switcher in the top right, as part of the pyadata-sphinx-theme.
• The only other thing the versions in the sidebar adds is a note about the last supported version for Python 2, but Python 2 is long end of life, so I think fine to remove this.
• Removing the versions from the sidebar removes a location where the version has to be manually updated every time a new release is done (e.g., [Doc]: Latest stable version in sidebar out of date #27863).

[timhoffm]

To prevent outdated information in the older docs, which still show this sidebar, I suggest to modify versions.html so that it does not show the version number for latest anymore:

and link docs and release notes to https://matplotlib.org/stable/

[timhoffm]

This PR should also remove the sentence

You will need to manually edit versions.html to show the released version.

from https://matplotlib.org/devdocs/devel/release_guide.html#build-and-deploy-documentation.