Add steps and details for running 'unprivileged' Elastic Agent

[kilfoyle]

These are ready for review now.

Main changes are in: Run Elastic Agent without administrative privileges

Smaller changes:

• Added an --unprivileged option to the elastic-agent install command.
• Added an Error when running Elastic Agent commands with sudo troubleshooting topic

Closes: #1077
Closes: elastic/elastic-agent#4705
Closes: elastic/elastic-agent#4653
Closes: elastic/elastic-agent#4647
Closes: elastic/elastic-agent#4675

Target: 8.15

[github-actions]

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[ycombinator]

Hi @kilfoyle, based on the comments in elastic/elastic-agent#4889, elastic/elastic-agent#4888, and elastic/elastic-agent#4887, we should include a section somewhere in this documentation about running Elastic Agent sub-commands using the same user as the one Agent uses when installing itself in --unprivileged mode. For specific suggestions on how to handle this scenario, see elastic/elastic-agent#4889 (comment).

[kilfoyle]

Hi @kilfoyle, based on the comments in elastic/elastic-agent#4889, elastic/elastic-agent#4888, and elastic/elastic-agent#4887, we should include a section somewhere in this documentation about running Elastic Agent sub-commands using the same user as the one Agent uses when installing itself in --unprivileged mode. For specific suggestions on how to handle this scenario, see elastic/elastic-agent#4889 (comment).

Thanks @ycombinator! I've added a prominent warning about this in the "Run Elastic Agent in unprivileged mode` section:

---

And also a new section on the troubleshooting docs page:

[kilfoyle]

Just for my own reference, updated the Command reference install command syntax and added this description:

[kilfoyle]

Thanks @ycombinator! It seems like we're near the finish line. Do you think there's anything else we should add or change?

[ycombinator]

Do you think there's anything else we should add or change?

Just #1087 (comment)