readthedocs: set explicit sphinx configuration

Not specifying a configuration is now deprecated and results in
permanent errors soon [1].

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

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

.readthedocs.yaml

@@ -1,5 +1,8 @@
 version: 2
 
+sphinx:
+  configuration: doc/conf.py
+
 python:
   install:
     - method: pip