docs: Update .readthedocs.yml to point to the sphinx config.

Now that there is only one sphinx project, the RTD config can be more
specific and stop relying on the old style config which is about to be
deprecated.

.readthedocs.yaml

@@ -5,6 +5,9 @@ build:
   tools:
     python: "3.8"
 
+sphinx:
+  configuration: source/conf.py
+
 python:
   install:
   - requirements: "requirements/edx/doc.txt"