[docs] remove references to route fallthrough

[Conduitry]

There are a few references to fallthrough routes that survived in the docs.

A couple of the references can be removed in a fairly naive way. One of the FAQ entries was removed because I don't see what it would make sense to replace it with. The reference in the Sapper migration docs about regex routes has been updated to refer to the new route matching feature.

I'm definitely open to more elegant ways of handling some of these changes, if anyone has ideas.

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Latest commit: 2292c36

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[Rich-Harris]

LGTM