[DOCS] Enhance the guidance to setup monitoring via Elastic Agents

[lucabelluccini]

Our docs at the page https://www.elastic.co/guide/en/elasticsearch/reference/current/configuring-elastic-agent.html#configuring-elastic-agent describe how to setup Elasticsearch monitoring via Elastic Agent Elasticsearch integration. It would be necessary to improve the docs to cover the following fact the "destination" cluster needs the Elasticsearch assets to be installed (which is done automatically when the destination cluster is the same where Fleet Server is running)

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[gbanasiak]

cc @alaudazzi

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)