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 don't really understand putting lack of ownership (borrows) under the banner of an over-arching ownership system. It would make sense if you were talking about use-after-move here. I don't think it's helpful to conflate terms together.