Add 'State using Signals' to tutorial

[rschristian]

Re: State - Preact Tutorial

We only cover state in class components & functional/hooks at the moment, would be good to get a mention of signals in there too. As this user mentions, our tutorial mirrors standard React at the moment, would be good to show the sort of experiences we can offer instead.

Might be a bit tricky if we show 3 options and then the rest of the tutorial is hooks-focused though. Will have a think, potentially we can support all 3?