Guide: pointers

Not the pointer guide, but the guide section on pointers.

Adding a section about patterns for after this, as well.
auto merge of #16119 : steveklabnik/rust/guide_pointers, r=brson

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.