Add readthedocs configuration file (#295)

As received by email, this configuration file will become mandatory:

> The Read the Docs build system will start requiring a configuration file v2 (.readthedocs.yaml)
> starting on September 25, 2023. We are sending weekly notifications about this issue to all impacted users,
> as well as temporary build failures (brownouts) as the date approaches for those who haven't migrated their projects.
1 file changed