docs: use a setInterval demo

[Rich-Harris]

as per #10745, it's better not to use a console.log example here. Instead, we can actually show why you might need to use a cleanup function

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[dummdidumm]

The new example is nice, but it would be even better to have a button there that changes the miliseconds ("go faster" that cuts the miliseconds in half maybe), so see the "reruns when miliseconds change" in action.