Add required sphinx conf path to .readthedocs.yaml

This is now required:

https://about.readthedocs.com/blog/2024/12/deprecate-config-files-without-sphinx-or-mkdocs-config/

Author: nicoddemus

.readthedocs.yaml

@@ -9,3 +9,6 @@ python:
   install:
     - method: pip
       path: .
+
+sphinx:
+  configuration: doc/conf.py