Document functionality to create GitLab issues from prometheus alerts

As discussed in gitlab-org/gitlab#506841 (comment 2260958973), we'd like to explore the option to create GitLab issues from prometheus alerts. The example here is to signal teams that some of their database tables are growing too large - so those will be rather long lived issues.

@stejacks-gitlab pointed me to https://gitlab.com/gitlab-com/runbooks/-/blob/master/libsonnet/alerts/walg-backups-alerts.libsonnet?ref_type=heads#L47-85.

It'd be great if we could gather what is available already here in this issue. I'll happily take it up to produce a docs update for this if that would be useful.