docs: requirements.txt: update to Sphinx 8.0.2 and sphinx-rtd-theme 3.0.0

As far as I can tell, the docs look fine with the new version.

This drops support for Python 3.9.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

docs/requirements.txt

-Sphinx==7.3.7
-sphinx-rtd-theme==2.0.0
+Sphinx==8.0.2
+sphinx-rtd-theme==3.0.0