[Community goal] Update contributor documentation

As the Ussuri goal we have an OpenStack project wide contributing
documentation template [1]. Nova has its own contributor documentation
so this patch adds the new contributing.rst according to the global
template and links the sections in it to the existing Nova contributor
documentation.

Also the top level CONTRIBUTING.rst template has been changed in the
cookiecutter repo[2]. So this patch updates the CONTRIBUTING.rst according
to the new template.

[1] https://opendev.org/openstack/cookiecutter/src/branch/master/%7b%7bcookiecutter.repo_name%7d%7d/doc/source/contributor/contributing.rst
[2] https://review.opendev.org/#/c/696001

Change-Id: Ia12cd4acbb63f192fcaf0790b72f8e977025a9dc
Story: #2007236
Task: #38541

doc/source/index.rst

@@ -222,6 +222,8 @@ Reference Material
 For Contributors
 ================
 
+* :doc:`contributor/contributing`: If you are a new contributor this should
+  help you to start contributing to Nova.
 * :doc:`contributor/index`: If you are new to Nova, this should help you start
   to understand what Nova actually does, and why.
 * :doc:`reference/index`: There are also a number of technical references on
@@ -234,6 +236,7 @@ For Contributors
    :hidden:
 
    contributor/index
+   contributor/contributing
    reference/index
 
 .. only:: html