site: clarify preserveWhitespace (#4341)

[sthagen]

• site: clarify preserveWhitespace

• Update 04-compile-time.md

Co-authored-by: Rich Harris [email protected]

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• It's really useful if your PR relates to an outstanding issue, so please reference it in your PR, or create an explanatory one for discussion. In many cases features are absent for a reason.
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Tests

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