[8.x] Add docs for advanced monitoring options (backport #1361)

[mergify]

This updates the Enable agent monitoring section of the docs, renamed to "Configure agent monitoring", and adds the advanced monitoring options shown below.

This also removes the Override the default monitoring port section since that's replaced in the UI by the new monitoring options.

target: 8.16
closes: #1333

I've also opened #1362 as a separate issue to add a few missing options to the Configure monitoring for standalone Elastic Agents page.

@cmacknz and @jen-huang It'd be nice to provide some additional detail for the /liveness endpoint if we can. For example, how to use it in Kubernetes to restart the container. Would you know where I can find that, or if there's something I can add a link to?

---

Advanced agent monitoring settings docs

---

---

Command reference docs

---

---

This is an automatic backport of pull request #1361 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.}