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.
Oh! I see now. That's because graves are a style thing, not a language thing. They're for formatting only. If I were talking about a variable named ownership, I would write

    `ownership`,

as well.