Re-arrange guessing game in the guide.

Now that the first section of the guide is largely in place, I've been thinking about it as a whole. One thing that's been clear as I've written it: the stuff here is so fundamental it's hard to do the 'build incrementally' bit I planned. So I'm moving the guessing game to the end of the current Guide.

In future sections of the guide, I hope to be more 'add this as we go.' I just think the material we're covering is a bit too fundamental now, and this will flow better.