[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

CONTRIBUTING.rst

@@ -1,16 +1,19 @@
-If you would like to contribute to the development of OpenStack,
-you must follow the steps in this page:
+The source repository for this project can be found at:
 
-   https://docs.openstack.org/infra/manual/developers.html
+   https://opendev.org/openstack/nova
 
-Once those steps have been completed, changes to OpenStack
-should be submitted for review via the Gerrit tool, following
-the workflow documented at:
+Pull requests submitted through GitHub are not monitored.
 
-   https://docs.openstack.org/infra/manual/developers.html#development-workflow
+To start contributing to OpenStack, follow the steps in the contribution guide
+to set up and use Gerrit:
 
-Pull requests submitted through GitHub will be ignored.
+   https://docs.openstack.org/contributors/code-and-documentation/quick-start.html
 
-Bugs should be filed on Launchpad, not GitHub:
+Bugs should be filed on Launchpad:
 
    https://bugs.launchpad.net/nova
+
+For more specific information about contributing to this repository, see the
+Nova contributor guide:
+
+   https://docs.openstack.org/nova/latest/contributor/contributing.html