Replace deprecated docs job in docs-on-readthedocs template
Replace deprecated docs job in docs-on-readthedocs template

Change docs-on-readthedocs project template from using deprecated
build-openstack-sphinx-docs job for check and gate to openstack-tox-docs
job.

Change-Id: I6105e584aee852672b96bd2b3ad4424a6d332e76

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs