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.
Since 'reference' already means something in Rust, I was trying to be generic. I'm pretty sure that you came up with the idea, actually, but maybe my brain made that up :)