Document Elastic Agent Installation scenarions for K8s observability

Currently Elastic Agent Installation for K8s Observability refers to only daemonset installations like in [managed](https://github.com/elastic/elastic-agent/blob/main/deploy/kubernetes/elastic-agent-managed-kubernetes.yaml#L3) and [standalone](https://github.com/elastic/elastic-agent/blob/main/deploy/kubernetes/elastic-agent-standalone-kubernetes.yaml#L667) scenarios.

This story will track the documentation changes needed to guide users to alternative deployment installation of elastic agent for cluster wide metrics. The goal is to guide elastic agent users on such not so trivial steps and to explain to them when its scenario can be chosen

## Related links:

- This is the [repo](https://github.com/elastic/ingest-docs/tree/main/docs/en/ingest-management/elastic-agent) where relevant elastic-agent pages are located
- [installation-methods-of-elastic-agent](https://github.com/elastic/elastic-agent/blob/main/docs/elastic-agent-ksm-sharding.md#installation-methods-of-elastic-agent)
- [proposed-agent-nstallations-for-large-scale](https://www.elastic.co/guide/en/fleet/current/scaling-on-kubernetes.html#_proposed_agent_installations_for_large_scale)