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.
hm, they don't have unique ownership, though, because it can be transferred. "Boxes own their contents" is better. the tough thing for me when discussing this is that there's no word for the contents of a box.

other thing: in the example below, I can't tell what the type of `y` is (like, I actually don't know at all). It really helped me when learning Rust to have a lot of explicit type annotations.