[Request] Document Fleet Server start up in Fleet or k8s quick-start guides

## Description
When we added Fleet Server with 7.13 release, we didn't initially appreciate (in my team) that it was more of a stack component than just a part of Agent (on the edge) or Fleet (in Kibana). We need to prioritize adding start up commands for Fleet Server anywhere it can be started (self managed, Cloud, ECE, ECK) - this ticket is about k8s / ECK.

*(Describe what needs to be documented. Link to relevant issues.)*
https://www.elastic.co/guide/en/cloud-on-k8s/master/k8s-quickstart.html
I checked here, and I think customers would expect this to have a 'Fleet Server' topic listed, as it has one for Elasticsearch and one for Kibana.

## Collaboration
Should this go to a different team (who covers ECK, overall) or no?

* [ ] The docs team will lead producing the content
* [ ] The product team will provide the initial content and the docs team will edit / review
* [ ] The docs team will define with product team the structure and location, and the product team will provide the initial content
* [ ] Other (please describe)

**Contact Person:**
@jlind23

## Suggested Target Release
7.16

## Stakeholders
Security Team, O11y Team, customers who use ECK / K8s

## Notes
I do see the overall 'Fleet Architecture' docs ticket here:
https://github.com/elastic/ingest-docs/issues/127
which references k8s briefly, but I don't think it had intentions to fully document how to start up the Fleet Server in a k8s pod / cluster, which is what this ticket is about.