[9.18] Add configuration for readthedocs.org
Backport of MR !8101 (merged)
Passing build: https://readthedocs.org/projects/bind9/builds/21335409/
Edited by Nicki Křížek
Backport of MR !8101 (merged)
Passing build: https://readthedocs.org/projects/bind9/builds/21335409/