doc: temporarily pin Sphinx requirement

Seems the latest Sphinx update has caused more issues ...

7.2.2 doesn't work with the notfound extension (see
readthedocs/sphinx-notfound-page#219 ).
When restricting Sphinx to <7.2.0, RTD uses 6.2.1 though, which
is incompatible with a Furo change (see
pradyunsg/furo#693 ).

Therefore, restrict Sphinx to 7.1.2 until the issues are resolved.

Signed-off-by: Ruth Fuchss <ruth.fuchss@canonical.com>

doc/.sphinx/requirements.txt

@@ -16,7 +16,7 @@ pytz
 requests
 six
 snowballstemmer
-Sphinx<7.2.0
+Sphinx==7.1.2
 sphinx-autobuild
 sphinxcontrib-applehelp
 sphinxcontrib-devhelp