Update requirements.txt

bump `sphinx-rtd-theme` in requirements for docs build, pin min version of `sphinx-argparse` in attempt to resolve build failurea similar to the one described in this issue: sphinx-doc/sphinx-argparse#57

docs/requirements.txt

@@ -1,7 +1,7 @@
 jinja2==3.1.2 # https://github.com/readthedocs/readthedocs.org/issues/9037#issuecomment-1077818554
 Sphinx==5.3.0 #override sphinx pinning done by RTD: https://docs.readthedocs.io/en/stable/build-default-versions.html#external-dependencies
-sphinx-argparse
-sphinx-rtd-theme==1.1.1
+sphinx-argparse>=0.5.2
+sphinx-rtd-theme>=1.3.0,<2 # pinning to <2.0 for now until dependency incompatibilities are resolved
 PyYAML==6.0
 mock==5.0.1
 recommonmark