Set up the doc infra and add doc job

devstack-plugins-ceph does not have the doc infra set
up and no doc publish job.

This is need to add the contributor doc as per the ussuri
community goal.

Change-Id: Ib62c34b52b28b753f7f3046d51492fd8d9fb6193

tox.ini

@@ -23,3 +23,12 @@ commands = bash -c "find {toxinidir}          \
           -wholename \*/lib/\*                \
          \)                                   \
          -print0 | xargs -0 bashate -v -iE006"
+
+[testenv:docs]
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
+commands =
+  rm -rf doc/build
+  sphinx-build -W -b html doc/source doc/build/html
+whitelist_externals = rm