Update pullmirroroverdue runbook
From production#802 (closed), we had to use a slide that was prepared for CREATE. Specifically around checking capacity. Need to update the runbook with this info since this step played a key role in troubleshooting the issue with getting the stuck jobs going again
Slides with background: https://docs.google.com/presentation/d/17BTT6M6RyNRckV4wTt-dr07nIfBvD325_xVBoLtSoPM/edit#slide=id.g4a8e7748ba_1_12
Runbook: https://gitlab.com/gitlab-com/runbooks/blob/master/troubleshooting/large-pull-mirror-queue.md
Edited by Andreas Brandl