Add docs for running unprivileged agent as existing user/group

[kilfoyle]

This adds docs for running Elastic Agent unprivileged mode, as a specific user or group (e.g. under an Active Directory account).

Targets: 8.x
Closes: #1503

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Update to Run Elastic Agent without administrative privileges page:

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Update to Command Reference elastic-agent install command --unprivileged option:

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Update to Command Reference elastic-agent unprivileged command:

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[mergify]

This pull request does not have a backport label. Could you fix it @kilfoyle? 🙏
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.

[michalpristas]

this looks good, many thanks @kilfoyle

[lcawl]

One comment of curiosity, otherwise LGTM