[8.x] Add Fleet Server install steps on K8s and Docker (backport #1184)

[mergify]

This updates the docs with the extra steps required to install Elastic Agent as a Fleet Server in containerized environments.

On Kubernetes

• On the page "Run Elastic Agent on Kubernetes managed by Fleet" this updates "Step 3: Enroll Elastic Agent to the policy" to add the following:
  

In Docker

• On the page "Run Elastic Agent in a container" this updates "Step 4: Run the Elastic Agent image" to add the following:
  

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Rel: #1075

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This is an automatic backport of pull request #1184 done by [Mergify](https://mergify.com).

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