Add a .readthedocs.yml configuration file

[jhedstrom]

See https://blog.readthedocs.com/migrate-configuration-v2/

[AlexSkrypnyk]

Will be addressed as part of the v3.x documentation update (see #312).

[AlexSkrypnyk]

Thanks for raising this back in 2023! Circumstances have shifted since - the standalone docs tree has been removed entirely. There was not enough content there to justify the ReadTheDocs setup, so instead of migrating the config, I have consolidated everything into the README and CONTRIBUTING files at the project root. Both render directly on GitHub, so consumers no longer need to leave the repository or rely on a third-party service to read the docs. Closing this out - the underlying need is resolved.