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.
It should say what `Rc` and `Arc` stand for here -- I had no idea that an `Arc` was an 'atomically reference counted' pointer until I asked @kmrhb