[ussuri][goal] Update contributor documentation

This patch updates/adds the contributor documentation to follow
the guidelines of the Ussuri cycle community goal[1].

[1] https://governance.openstack.org/tc/goals/selected/ussuri/project-ptl-and-contrib-docs.html
Story: #2007236
Task: #38554

Change-Id: I870955fda7ee6e0d7809faa309bbc31c2ddd653a
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 6694022..7923cb8 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -151,6 +151,13 @@
 with devstack, and help us by :doc:`contributing to the project
 <hacking>`.
 
+If you are a new contributor to devstack please refer: :doc:`contributor/contributing`
+
+.. toctree::
+   :hidden:
+
+   contributor/contributing
+
 Contents
 ++++++++