Add config path to RTD config (#1254)

.readthedocs.yml

@@ -8,6 +8,7 @@ version: 2
 # Don't publish the docs if there are any warnings, as they might be relevant
 # https://github.com/readthedocs/readthedocs.org/issues/6837#issuecomment-607229633
 sphinx:
+  configuration: docs/source/conf.py
   fail_on_warning: true
 
 # Formats of the documentation to be built