explicit configuration file for readthedocs sphinx (#681)

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

Author: glyph

.readthedocs.yaml

@@ -3,6 +3,7 @@ version: 2
 sphinx:
   # We want to fail as this is also our CI check for the docs.
   fail_on_warning: True
+  configuration: docs/conf.py
 
 # We don't need PDF and epub for the docs.
 formats: []