[Stack Monitoring] Update monitoring template indices docs

[matschaffer]

The docs at https://www.elastic.co/guide/en/elasticsearch/reference/current/config-monitoring-indices.html only apply to internal collection and make no mention of the data streams introduced for metricbeat monitoring in 8.0.

We should add some mention about that.

Note that until elastic/elasticsearch#84149 is resolved we can't provide any customization of the embedded templates.

If that issue is closed by the time we pick this issue up, we can include override info the docs. If it's still open we should call out that it can't be customized.

[dedemorton]

Closed by elastic/elasticsearch#94348. I've added a short section about elastic agent with pointers to the related docs about data streams. If those docs need more work, feel free to open another issue, but I think we should point to the related docs rather than duplicating info under the stack monitoring docs.