Docs: Add “Is [some particular site] usable in Ladybird” to FAQ

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I’m not wedded to the particular wording of either the answer or the question here. But we get this question (in some form) frequently enough from people showing up on Discord — for example, see https://discord.com/channels/1247070541085671459/1247070543136690270/1315167861567914047 from just a few hours ago today — that it seems to merit being included in the FAQ.