Import GitLab integration with our infrastructure documentation

On https://salsa.debian.org/tails-team/gitlab-migration/-/wikis/sysadmin/gitlab-integration, @zen described the constraints we had to adjust our infrastructure from Redmine to GitLab, and documented the resulting setup, which is now live.

We should import the relevant pieces of that doc somewhere, e.g. under https://gitlab.tails.boum.org/tails/tails/-/tree/master/wiki/src/contribute/working_together/roles/sysadmins where similar doc currently lives.

• Include information about the root user, including that it doesn't have notifications enabled. (ref)