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.
"in an `Rc`" i.e. the thing wrapping the car is the Rc, to create an `Rc<T>`, we're *not* taking an `Rc<T>` for some `T` and putting that around the car.