docs: add .readthedocs.yaml configuration file

Read the Docs has started requiring a .readthedocs.yaml configuration
file to be present in the repository in order to build the
documentation.

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

According to the timeline we have until 2023-09-25 to migrate to the new
configuration file, but we've already had some builds fail during their
48-hour brownout on 2023-09-04.