Add small doc about running a standalone agent in otel mode

[pchila]

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

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @pchila? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[kilfoyle]

@pchila I think this looks great! I've added a few small edits in ca5248e, mostly to match other docs format, but feel free to revert anything at all. Thanks a lot for putting this together! Please let us know when you're ready to join the docs team. :-)

[kilfoyle]

Here's a preview:

[pchila]

@kilfoyle Thanks for the review, edits in ca5248e look great!
Now I need another review (or a force merge) since GH is detecting that you were the last pusher so I need someone else to approve the PR 😓

[kilfoyle]

I'm glad you like the changes! I'll find a reviewer so we can get this merged.

[marciw]

LGTM 🐶