doc: ci: pin to Sphinx < 8.2.0

Sphinx 8.2.0 was just released but doesn't seem to work all that well
for PDF generation. Stick to 8.1.x for now.

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

Author: kartben

doc/requirements.txt

@@ -1,6 +1,6 @@
 # DOC: used to generate docs
 
-sphinx
+sphinx<8.2.0
 sphinx_rtd_theme~=3.0
 sphinx-tabs
 sphinxcontrib-svg2pdfconverter