Create `.readthedocs.yaml` v2 configuration file

[glemieux]

Currently the build configuration settings for the fates-doc are handled in the rtd web interface. We will need to include a .readthedocs.yaml configuration file as a requirement to build the documentation for PRs starting September 25, 2023. See the following blog entry for details: https://blog.readthedocs.com/migrate-configuration-v2/. Note that the fates-users-guide is all set in this regard and we can probably simply copy/paste that config file here.