Trac #34092: Docbuild workflow broken because furo is too new

As seen for example in
https://8f7574169b6743890bc32b58d9d2afcb4a5d6375--sagemath-
tobias.netlify.app/
{{{
ERROR: This page was generated by inheriting from "layout.html".

Furo (the documentation theme that this site is generated with) does not
support this, and has generated this page. If you are the author of this
documentation, please look into why this page was generated using the
"layout.html" template and fix the root cause.

Debugging information, if you need to report this error somewhere:
Sphinx: 4.4.0
Furo: 2022.06.21

pagename: index
}}}

As a workaround, we back out the use of furo on the docbuild CI.

URL: https://trac.sagemath.org/34092
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Kwankyu Lee

.github/workflows/doc-build.yml

@@ -24,7 +24,6 @@ jobs:
           # Reuse built SAGE_LOCAL contained in the Docker image 
           ./bootstrap
           ./configure --enable-build-as-root --prefix=/sage/local --with-sage-venv --enable-download-from-upstream-url
-          make furo
 
       - name: Build
         run: make doc-html