docs: put restrictions on Sphinx versions

Sphinx has released v6.0.0 (and several versions beyond) in the past
few weeks, which unfortunately broke some aspects of the ReadTheDocs
Sphinx theme that we use (sphinx-rtd-theme).  See
readthedocs/sphinx_rtd_theme#1403 for some
details.

Temporarily put some restrictions on the Sphinx and sphinx-rtd-theme
versions that we use so that everything renders correctly.  Once
sphinx-rtd-theme >= v1.2.0 is available (which correctly handles
Sphinx >= v6.0.0), we can remove the version restriction, and go back
to using whatever versions are current.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

Author: jsquyres

docs/requirements.txt

@@ -1,5 +1,5 @@
 --use-feature=2020-resolver
-sphinx>=4.2.0
+sphinx>=4.2.0,<=5.3.0
 recommonmark
 docutils
-sphinx-rtd-theme
+sphinx-rtd-theme<1.2.0