[Doc] Add documentation for contributing users
Changes follow the draft in https://docs.google.com/document/d/1ka1rBASGXDJ1ibU7O-5S6ue_na0EXZicYi4GZCtYnA0/edit?usp=sharing
Git-LFS is now available on our GitLab instance.
Please note that the quota check is now in blocking mode. More informations about quotas: https://gitlab.inria.fr/siteadmin/doc/-/wikis/faq#quota-management-policy-to-monitor-disk-space-by-project-on-gitlab
Changes follow the draft in https://docs.google.com/document/d/1ka1rBASGXDJ1ibU7O-5S6ue_na0EXZicYi4GZCtYnA0/edit?usp=sharing