Add config file for Read the Docs

[hugovk]

The Read the Docs builds have started to fail:

Error
Problem in your project's configuration. No default configuration file found at repository's root.

https://readthedocs.org/projects/pythondotorg/builds/
https://readthedocs.org/projects/pythondotorg/builds/22548846/

That's because Read the Docs now requires projects to have .readthedocs.yaml

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

---

Earlier builds included the readthedocssinglehtmllocalmedia builder, which created a downloadable zip of the HTML files:

https://readthedocs.org/projects/pythondotorg/builds/21499586/

I didn't add this back. Do we need it?

[malemburg]

LGTM.

[willingc]

Thanks @hugovk