[8.15] Fixes for 'unprivileged' agent docs (backport #1200)

[mergify]

This updates the Run Elastic Agent without administrative privileges page based on suggestions in #1197

Closes: #1197

Preview pages:

• Run Elastic Agent without administrative privileges
• elastic-agent install command

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Fixes mapped to the "Unprivileged mode docs review" issue

1. Updates the warning about running commands in unprivileged mode to be more clear.

2. Describes the privileged and unprivileged commands for changing an agent's running mode.

3. Nothing to do for now.

4. Adds steps to view an agent's running status (privileged vs unprivileged) in the "Agent details" UI.

5. Nothing to do for now.

Additional fix

6. Updates the Command Reference to the --privileged parameter to the elastic-agent install command.
   @ycombinator Do you think elastic-agent privileged and elastic-agent unprivileged should also be documented as full commands, similar to elastic-agent inspect, elastic-agent status, etc.? From Add unprivileged and privileged subcommand to Elastic Agent elastic-agent#4621 it seems so. That is, they're actual commands rather than just parameters on the elastic-agent install command. However, these are also referred to as "subcommands" so I'm kind of puzzled. :-)

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This is an automatic backport of pull request #1200 done by [Mergify](https://mergify.com).

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