readthedocs config: add sphinx configuration key

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

.readthedocs.yaml

@@ -14,3 +14,6 @@ python:
    install:
       - requirements: requirements.txt
       - requirements: docs/requirements.txt
+
+sphinx:
+  configuration: docs/conf.py