[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: #38537
Change-Id: Ib60141ca2274780f9d1ed50e4e9b3a50ba6c3d7c

openstack/mistral-specs