gh-143284: Temporarily install Sphinx<9 to fix Chinese search (GH-143286)

(cherry picked from commit 0aedf2f)

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: hugovk

Doc/requirements.txt

@@ -7,7 +7,7 @@
 # won't suddenly cause build failures. Updating the version is fine as long
 # as no warnings are raised by doing so.
 # Keep this version in sync with ``Doc/conf.py``.
-sphinx~=9.0.0
+sphinx<9.0.0
 
 blurb