Fix documentation build

The contributing.rst file was referencing a non-existing path, and
index.rst referenced a non-existing usage.rst file.

Added build-openstack-sphinx-docs job to test doc changes

Change-Id: Ie08e6b1a49a69fedb0fe1ddcf0225d0018aa65b4
diff --git a/.zuul.yaml b/.zuul.yaml
index 359107b..1d7e323 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -108,9 +108,11 @@
         - neutron-tempest-plugin-designate-scenario
         - neutron-tempest-plugin-dvr-multinode-scenario
         - neutron-tempest-plugin-scenario-linuxbridge
+        - build-openstack-sphinx-docs
     gate:
       jobs:
         - neutron-tempest-plugin-api
+        - build-openstack-sphinx-docs
 
 - project:
     templates: