Update readthedocs config

[kandersolar]

• Closes #xxxx
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

RTD doesn't like this project's configuration file. See the warnings on this build: https://readthedocs.org/projects/solarfactors/builds/21172217/

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This PR updates the configuration file to the current recommended state.

[echedey-ls]

Any reason this got delayed? Porting pvlib org packages to share a common infrastructure seems like a priority in the long term.

[kandersolar]

I probably just never came back to merge it after waiting for the CI build to complete. Thanks for the reminder @echedey-ls :)