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:
diff --git a/doc/source/contributing.rst b/doc/source/contributing.rst
index 2aa0707..1728a61 100644
--- a/doc/source/contributing.rst
+++ b/doc/source/contributing.rst
@@ -1,4 +1,4 @@
 ============
 Contributing
 ============
-.. include:: ../../../CONTRIBUTING.rst
+.. include:: ../../CONTRIBUTING.rst
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 1bc38e1..91388a9 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -14,7 +14,6 @@
 
    readme
    installation
-   usage
    contributing
 
 Indices and tables