Add troubleshooting section to pod-to-pod-multicluster for required metadata in secret

[Majkel1999]

Adding Troubleshooting section to the pod-to-pod-multicluster task, as the required metadata for secrets in control plane namespace is not documented.

References:

• https://github.com/linkerd/linkerd2/issues/11542
• https://github.com/linkerd/linkerd2/blob/ce950d17c60eab8c7c2a3e67c130241f96056ac2/controller/api/destination/watcher/cluster_store.go#L63C1-L67

Signed-off-by: @Majkel1999 [email protected]

[alpeb]

The linkerd multicluster link command should be used to retrieve the Link resource and the other associated resources (including those Secrets) to have multicluster communication properly work. I understand it's not always possible to plug a call to this command in all deployment pipelines, but the manifests to use should still be sourced from this command which provides the ultimate source of truth. So I don't see why we need to single out specific metadata that is already provided, and that is required just like many other pieces of metadata in other linkerd resources.