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.
I think this section may be clearer as:

> The `box` keyword creates a `Box` (specifically `Box<int>` in this case) by allocating a small segment of memory on the heap with enough space to fit an `int`. But where does the code say the box is deallocated?

Or, the last sentence could be `But when is the box deallocated?`.

(This is continuing @mdinger's point, but is a new thread so it's not lost in other old discussion.)