Updates security developer workflow
What does this MR do?
Modifies security developer workflow to indicate security development should be done on GitLab security (for GitLab security fixes) and on dev.gitlab.org (for Omnibus security fixes).
It also re-orders the doc to make it more clear.
Related to gitlab-com/gl-infra&121 (closed)
Should be merged before the announcement
Edited by 🤖 GitLab Bot 🤖