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.
FWIW, too many times is the worst case: it is undefined behaviour and can lead to memory corruption and security holes, while a leak is just a piece of memory that is never touched again.