Add group to list of groups for onboarding TWs

This MR updates the list of groups a new TW needs access too. Essentially to avoid this problem in the future: gitlab-runner!3213 (comment 742106750).

@marcel.amirault seemed to have a lot of context here for why there is two groups required for a TW, so I'll ask him to chime in here.

@dianalogan @cnorris @sselhorn WDYT?

Also added latest Markdownlint configuration from gitlab project.