Workflow documentation is now in infra-manual

Replace URLs for workflow documentation to appropriate parts of the
OpenStack Project Infrastructure Manual.

Change-Id: I1ba19377ed279ccb9f8d698b15678c528cbeade3

CONTRIBUTING.rst

@@ -1,14 +1,13 @@
 If you would like to contribute to the development of OpenStack,
-you must follow the steps in the "If you're a developer, start here"
-section of this page:
+you must follow the steps in this page:
 
-   http://wiki.openstack.org/HowToContribute
+   http://docs.openstack.org/infra/manual/developers.html
 
 Once those steps have been completed, changes to OpenStack
 should be submitted for review via the Gerrit tool, following
 the workflow documented at:
 
-   http://wiki.openstack.org/GerritWorkflow
+   http://docs.openstack.org/infra/manual/developers.html#development-workflow
 
 Pull requests submitted through GitHub will be ignored.