[sphinx] Add an explicit sphinx.configuration value

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

Author: Pierre-Sassoulas

.readthedocs.yml

@@ -1,5 +1,8 @@
 version: 2
 
+sphinx:
+  configuration: docs/conf.py
+
 python:
    install:
      # Without this, sphinx can't find pluggy's version.