STYLE: Specify ReadTheDocs custom domain configuration

[thewtex]

Specify the Read The Docs custom domain in the documentation config file.

Read the Docs is deprecating Sphinx context injection at build time starting Monday, Oct 7, 2024, so any custom domain specified in the Read the Docs admin needs to be defined in the configuration file.

Documentation: https://about.readthedocs.com/blog/2024/07/addons-by-default/