Add configuration for readthedocs.org
readthedocs.org is switching to in-repo configuration file in favor of the deprecated web interface as it allows more flexibility.
This also fixes our recent doc build issues, as we're switching to a newer Python which is required by Sphinx.
See https://blog.readthedocs.com/migrate-configuration-v2/
readthedocs build: https://readthedocs.org/projects/bind9/builds/21300032/