Test and document association with external ref for general use

[thbkrkr]

• Support custom Secret for associating external Elastic resources not managed by ECK #5240

was implemented for the purpose of Stack Monitoring but we designed it to be usable for all associations.

Today, it is tested and documented only for Stack Monitoring.

To make it generally usable for all associations, it would be necessary to:

• test all combinations
• document the feature

[barkbay]

👍 on this one as https://www.elastic.co/guide/en/cloud-on-k8s/current/k8s-kibana-es.html#k8s-kibana-external-es feels outdated.