[9.16] Add configuration for readthedocs.org
Backport of MR !8101 (merged)
Passing doc build: https://readthedocs.org/projects/bind9/builds/21335410/
Edited by Nicki Křížek
Backport of MR !8101 (merged)
Passing doc build: https://readthedocs.org/projects/bind9/builds/21335410/