As discussed in #33309 comment:28, we add Furo (https://pradyunsg.me/furo/) as a possible theme for the documentation.
After gaining some experience with it, the goal would be to make it the default theme for sage's docs.

To test, run

make furo
make doc-clean; make doc-uninstall; make

To go back to the classic sage theme, uninstall furo using ./sage -pip uninstall furo.

Known issues (to be fixed as follow-ups/upstream):

• Reference manual index page and many sub-pagaes don't have a table of contents on the left https://516dfe85b21eafae4f042320e5dcf61ad14fe780--sagemath-tobias.netlify.app/reference/
• Index page is missing/not working https://516dfe85b21eafae4f042320e5dcf61ad14fe780--sagemath-tobias.netlify.app/
• sage logo in the side bar should link to the top of the Sage manuals

Depends on #25833
Depends on #33740

CC: @kwankyu @strogdon @haraldschilly @mkoeppe

Component: documentation

Author: Tobias Diez

Branch/Commit: aec8fec

Reviewer: Kwankyu Lee

Issue created by migration from https://trac.sagemath.org/ticket/33601