This package was removed from the OpenStack requirements with [1] so
keeping it in the doc/requirements.txt file is causing failure in the
requirements-check job.
As we want to use this theme to build our docs, lets keep it in the
readthedocs_requirements.txt file which isn't checked by the
requirements-check job at all.
This patch also changes docs and releasenotes configs in the way that it
will fallback to one of the default themes provided by sphinx.
Finally it makes some additional changes to the releasenotes conf.py
file to make build of the releasenotes working fine.
[1] https://review.opendev.org/c/openstack/requirements/+/893665
Change-Id: Iff03b8596a9e772940fd959efa7cbf80a68e9eab
c24dd0c244