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.
Possible point of confusion for someone who has *seen* borrows but doesn't get them yet: passing a Box into a function then back out again is not the same as borrowing, since you can have multiple immutable borrows at the same time.  This might not be a problem, and I can't think of different phrasing that doesn't devolve into a massive run-on sentence. :P