Make configuration file explicit

Read the Docs has deprecated the use of Sphinx projects without an
explicit path to the configuration file. This adds the path to
`.readthedocs.yaml`.

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

Author: andersfagerli

.readthedocs.yml

@@ -15,6 +15,7 @@ python:
 sphinx:
   builder: html
   fail_on_warning: true
+  configuration: docs/conf.py
 
 # Also build PDF & ePub
 formats: