Add gate-docs job to gear.

Change-Id: If629f322c965f1cc88f32fd0a3efdc6e680ddb2b
Reviewed-on: https://review.openstack.org/26559
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins