Incrementalize tutorial doc

Right now the tutorial is coherently designed, tested, and even documented. However, it doesn't build up in a way that's very beginner friendly. It establishes glom's value and then immediately uses it at an intermediate level.

I'd like it if it was a bit more drawn out to use basic features first and then add a multi-line Coalesce as the finisher. The announcement blog post does a better job of this, but doesn't go far enough before jumping ahead to T.