Describe how to access monitoring cluster via CCS

Per https://github.com/elastic/elasticsearch/pull/42655#pullrequestreview-243954062

> One thing to consider is that technically users can configure filebeat to the production cluster and setup CCS and the monitoring UI will be able to read those indices. Do we want to include this in the docs?

This seems to be another method of configuring monitoring in a production environment, which should be added to:
https://www.elastic.co/guide/en/elastic-stack-overview/master/monitoring-production.html