Add docs for cleaning up GKE clusters

The following discussion from !499 (merged) should be addressed:

• @tauriedavis started a discussion: (+1 comment)

  Should we add docs for removing clusters here or somewhere else?

  1. Log into https://console.cloud.google.com/
  2. Visit Kubernetes Engine -> Clusters
  3. Remove unused clusters
  4. Visit Network Services -> Load balancers
  5. Open every one of the TCP load balancers in new tabs
  6. Check for the yellow warning next to the nodes list saying the nodes they point to no longer exist
  7. Delete the load balancer if it has no green ticks and only yellow warnings next to the nodes

  A couple questions:

  1. What about TCP/UDP? Should those be checked as well?
  2. What about load balancers that say This load balancer has no health check, so traffic will be sent to all instances regardless of their status?

Also docs for cleaning up unused disks when you're done:

1. Visit Compute Engine -> Disks
2. Filter In use by:''
3. Delete disks that correspond to your cluster (probably you may want to filter by name to name: gke-dgriffith* to find your own ones). Also see automated command for cleaning out our unused disks for QA in gitlab-qa#298 (comment 104822741)