Add note about unprivileged mode on 'install' tabs

[kilfoyle]

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]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[kilfoyle]

@elasticmachine run docs-build

[kilfoyle]

@elasticmachine run docs-build rebuild

[kilfoyle]

Closing in favour of #1144

The docs CI checks don't seem to recognize that #1087 is merged with it's content already included in this PR.