Add explicit docs requirements

ReadTheDocs no longer installs rtd-theme by default in its virtual environments, so we
need to explicitly install it.

Author: nicoddemus

.readthedocs.yml

@@ -11,3 +11,4 @@ sphinx:
 python:
   install:
     - path: .
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,2 @@
+sphinx
+sphinx-rtd-theme