Add warning in the Quick Start guides about Safari breaking Copy to Clipboard

[abey79]

What

For security reason, Safari doesn't let use copy text from our Web viewer, see:

• Copy doesn't work on Safari emilk/egui#3480

This PR detects if we are running on Safari and adds a warning to the top of the Quick Start guides.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

• PR Build Summary
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[emilk]

Suggested change

	#[cfg(target_arch = "wasm32")]
	let result = web_sys::window()?.navigator().user_agent().ok();
	#[cfg(not(target_arch = "wasm32"))]
	let result = None;
	result
	}
	#[cfg(target_arch = "wasm32")]
	eframe::web::user_agent()
	#[cfg(not(target_arch = "wasm32"))]
	None
	}

[abey79]

That doesn't work.

[abey79]

This is the most compact I can think of:

fn user_agent() -> Option<String> {
    #[cfg(target_arch = "wasm32")]
    return eframe::web::user_agent();

    #[cfg(not(target_arch = "wasm32"))]
    None
}