Update `.readthedocs.yaml` with Sphinx key

As discussed in a [post on the Read the Docs blog](https://about.readthedocs.com/blog/2024/12/deprecate-config-files-without-sphinx-or-mkdocs-config/) projects can no longer be built if `.readthedocs.yaml` doesn't have a `sphinx.configuration` key. 