Specifically html/en/index.html page is now based on furo's page.html for furo theme.

Along the way, we fix here and there so that

• pdf docs also build
• logo links to the documentation root
• search page works
• white sage logo is transparent

Hence our doc in furo theme is now quite functional.

CC: @yuan-zhou @tobiasdiez

Component: documentation

Author: Kwankyu Lee

Branch/Commit: 73e5aa3

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/34252