[community goal] Update contributor documentation
[community 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: #38550
Change-Id: I4c6cb07fc0fab0180cea3d5f6284aa17a7c7dff7

openstack/pymod2pkg