fixes sphinx.configuration key is missing

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

Author: spjuhel

.readthedocs.yml

@@ -22,3 +22,6 @@ python:
 
 formats:
   - pdf
+
+sphinx:
+  configuration: doc/conf.py