Add non-voting docs job for git-review

To test https://review.openstack.org/#/c/138160 , add a non-voting
job for building the documentation. Once this works, we can mark
the job voting and publish the docs.

Change-Id: I1d155de1a3abbb496e6954f373d919fddf55a013
Merge "Add non-voting docs job for git-review"