Merge branch 'doc/17753-gitlab-config' into 'master'

GitLab: document gitlab-config repo and what it's useful for

Closes #17753

See merge request !40

wiki/src/contribute/working_together/GitLab.mdwn

@@ -436,6 +436,22 @@ instance"]].
 The Tails [[system administrators|working_together/roles/sysadmins]]
 administrate this GitLab instance.
 
+## Configuration of GitLab
+
+The configuration of our GitLab instance lives in the private [[!tails_gitlab
+tails/gitlab-config]] GitLab project.
+
+If you have access to this project, you can propose configuration changes:
+push a topic branch and submit a merge request.
+
+This can be useful, for example:
+
+ - to modify group membership when someone joins or leaves a team
+ - to propose new [[!tails_gitlab groups/tails/-/labels desc="group labels"]]
+   shared by all our GitLab projects
+ - to propose a new project under the `tails/` namespace, ensuring our common
+   project settings & permission model are applied
+
 <a id="access-control"></a>
 
 ## Access control