Add note about unprivileged mode on 'install' tabs (take2)

[kilfoyle]

This is a second attempt since the first attempt fails CI checks.

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This updates the Install standalone Elastic Agents docs page, so the install tabs for macOS, Linux, DEB and RPM would have the following note:

@ycombinator I know that in this comment you mentioned only RPM and DEB, but I think this note should apply to all four operating systems, right, since they all show sudo in the install command? I tried the phrase the text so that it applies to all four.

Rel: #4125

[github-actions]

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[ycombinator]

Let's wait to merge this PR until we have a resolution to elastic/elastic-agent#4125 (comment), in case it has any docs impact.