[8.x] Add small doc about running a standalone agent in otel mode (backport #1572)

[mergify]

Quick documentation about configuring an installed, standalone Elastic Agent to run as OTel Collector (possible thanks to elastic/elastic-agent#5767).

Closes elastic/elastic-agent#4933

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This is an automatic backport of pull request #1572 done by Mergify.

[mergify]

Cherry-pick of 99836e0 has failed:

On branch mergify/bp/8.x/pr-1572
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 99836e0.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/ingest-management/elastic-agent/otel-agent-transform.asciidoc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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