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 started a pre-RFC on doc conventions, but haven't had the time to make it into a real one. There's at least one open GitHub Issue about it.=