[8.15] 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:
  

---

Rel: #1075

---

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

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}