Guide: pointers

This is the next section of the guide, and it's on pointers. It's not done yet, as I need to write the section on ownership and borrowing, but I figured I'd share the rest now, to get feedback on the rest of it while I take some time to get that right.
No insult taken!

I've been specifically dropping references to the theory like this previously in the guide. I like the idea of allowing people to be able to find out more, and by calling things out by name.

I agree that the `Box<T>` change could go, though, as it is awkward to include the name, even though it feels more complete.