[Doc] Add documentation for contributing users
Changes follow the draft in https://docs.google.com/document/d/1ka1rBASGXDJ1ibU7O-5S6ue_na0EXZicYi4GZCtYnA0/edit?usp=sharing
GitLab upgrade completed. Current version is 18.5.4
https://about.gitlab.com/releases/2025/09/18/gitlab-18-4-released/
https://about.gitlab.com/releases/2025/10/16/gitlab-18-5-released/
Changes follow the draft in https://docs.google.com/document/d/1ka1rBASGXDJ1ibU7O-5S6ue_na0EXZicYi4GZCtYnA0/edit?usp=sharing