Pin sphinx_rtd_theme to the previous version for now (#99)

[reviewed by @jabraham17]

It looks like a new version was pushed last night and that triggered a
deprecation warning with Sphinx in our nightly testing. I have an action
item to look into updating the various package versions we rely on in
other places, but for now pin to the older version

Passed the testing suite via CI testing

docs-requirements.txt

@@ -1 +1 @@
-sphinx_rtd_theme
+sphinx_rtd_theme==2.0.0