doc: Bump to Sphinx 7.1 and RTD theme 2.0

This moves to more recent versions of Sphinx and Read the Docs theme.
FWIW Sphinx 7.2.0 dropped support for Python 3.8 so we're stuck with 7.1
for now.

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

doc/requirements.txt

@@ -1,8 +1,8 @@
 # DOC: used to generate docs
 
 breathe>=4.34
-sphinx~=6.2
-sphinx_rtd_theme~=1.2
+sphinx~=7.1.0
+sphinx_rtd_theme~=2.0
 sphinx-tabs
 sphinxcontrib-svg2pdfconverter
 pygments>=2.9