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

Open the monitoring-production.html page linked in the issue and review the referenced pull request discussion. Document the CCS-based way to let Filebeat use the production cluster for monitoring, including the relevant setup context. Done means the production monitoring documentation describes this alternative clearly and consistently with the existing methods.

Written by the indexing model from the issue text.