[8.x] Add docs for running unprivileged agent as existing user/group (backport #1505)

[mergify]

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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This is an automatic backport of pull request #1505 done by [Mergify](https://mergify.com).

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