docs: adds a customer codename mapping note in the Dedicated maintenance window toil issue.
What
Adds a quick link to the customer mapping table in the GitLab Dedicated toil issue.
Why
This MR is aimed at reducing doubts for the SRE on-call about which tenant instance to operate on during maintenance window. It complements https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/merge_requests/671.