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.
It seems to me that the concept of ownership and borrowing are intimately intertwined. That is, when you're discussing how one handles types with ownership semantics, borrowing is one of the key tools to talk about.