[Request]: Obs quick starts need to mention the stack as a prereq

[dedemorton]

Description

Not sure how I overlooked this, but we should show ESS (of the stack) as a prereq for the quickstarts:

https://www.elastic.co/guide/en/observability/master/quickstart-monitor-hosts-with-elastic-agent.html
https://www.elastic.co/guide/en/observability/master/monitor-k8s-logs-metrics-with-elastic-agent.html

Resources

n/a

Which documentation set does this change impact?

Stateful and Serverless

Feature differences

n/a

What release is this request related to?

8.16

Collaboration model

The documentation team

Point of contact.

Main contact: @dedemorton

Stakeholders: