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.
dependency: the first usage of 'borrow' is here (even though it's in an error message), but it's not explained anywhere on this page