text: Title: explicit configuration file missing in readthedocs.yml
  Body: ### What is your issue?
  
  According to https://about.readthedocs.com/blog/2024/12/deprecate-config-files-without-sphinx-or-mkdocs-config/ 
  
  `configuration: doc/conf.py`
  
  needs to be added to .readthedocs.yaml:
  
  https://github.com/pydata/xarray/blob/76120781f2b2205cc37bd97d0b4ed13d78521d75/.readthedocs.yaml#L17-L19