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 also felt that it was strange to equate that pattern (returning references) to that of borrowing. Perhaps instead say something like "While this is perfectly valid it becomes unwieldy in practice. Therefore Rust introduces a concept to describe a handle ..."