Add requirements.txt for documentation.

.readthedocs.yaml

@@ -30,6 +30,6 @@ sphinx:
 # Optional but recommended, declare the Python requirements required
 # to build your documentation
 # See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
-# python:
-#   install:
-#     - requirements: docs/requirements.txt
+ python:
+   install:
+     - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1 @@
+sphinx-rtd-theme==2.0.0