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.
Eurgh, I always get upset by quoted strings including things that aren't really inside them... but particularly for technical terms (i.e. the concept is just `ownership`, not `ownership.`... the dot is irrelevant). Maybe I'm just too pedantic.