Document new URL format

Add a description of the new URL format for project documentation.
We've been getting a bunch of patches to modify URLs and it's hard
to know which ones are the canonical format since there are redirects
so that both the old and new URLs currently work.

Change-Id: I84136af8315f3421f7d7825fdb7d096f019a6e08

doc/source/contributor/documentation.rst

@@ -22,6 +22,29 @@ easy to find answers to.
 
     http://glance.openstack.example.org/v2/images
 
+* URLs for OpenStack project documentation
+
+  Each project's documentation is published to the following URLs:
+
+  - ``https://docs.openstack.org/$project-name/latest`` - built from master
+  - ``https://docs.openstack.org/$project-name/$series`` - built from stable
+
+  For example, the Glance documentation is published to:
+
+  - ``https://docs.openstack.org/glance/latest`` - built from master
+  - ``https://docs.openstack.org/glance/ocata`` - built from stable/ocata
+
+* URLs for OpenStack API Reference Guides
+
+  Each project's API Reference Guide is published to:
+
+  - ``https://developer.openstack.org/api-ref/$service-type``
+
+  For example, the Glance Image Service API Reference guide is
+  published to:
+
+  - ``https://developer.openstack.org/api-ref/image``
+
 Where to Contribute
 -------------------