Ownership guide

This is a work in progress, but this should get *extensive* review, so I'm putting it up early and often.

This is the start of a draft of the new 'ownership guide,' which explains ownership, borrowing, etc. I'm feeling better about this framing than last time's, but we'll see.
Nitpick: Since *boxes are allocated on the heap* refers to some memory location, then *where is the box deallocated?* is also suggestive of a *memory* location accordingly. You might want to reword so the deallocation location sounds like it's referring to line number or something similar.