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.
nit: there's some unnecessary repetition between this and the previous paragraph now. could replace it with just 

This means we get the benefits of manual memory management, but the compiler ensures that we don't do
something wrong. We can't forget to `free` our memory.