[Website]: Stack monitoring data will not appear if `monitoring.cluster_uuid` is not set

### Type of issue

Missing information

### What documentation page is affected

https://www.elastic.co/docs/reference/logstash/monitoring-with-elastic-agent

### What happened?

In case the monitoring is enabled, but you are using the _"Stack Monitoring"_ page to see Logsstash data, you need to configure the `monitoring.cluster_uuid`.
We discovered that in a case where the Logstash data would appear on the Logstash Dashboards, but not on the _"Stack Monitoring"_ page.

It would be important to leave a warning about this setting, since it can be important for anyone trying to see the data on another cluster.

### Additional info

_No response_