Docs: Conditional rendering

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Note to self: people keep asking how to do this, therefore our docs don't do a good job of explaining it.

After the docs are updated for 15.0.0, review what's there and probably create a new section. Seems like right now we don't even have a dedicated section for child.maybe. IIRC 15.0.0 introduces a new filterWith(signalOfBool) method, so that's sort of kind of another option, although it behaves differently. Maybe we need a similar operator for Laminar itself... There's an issue filed on that already.

child.maybe <-- myObservableOfOptionA

child <-- observable.map(... => if (...) createMyElement() else emptyNode)