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.
"transfer" is a nice word to use to describe this ownership behaviour (don't feel obliged to use it, but I certainly often describe things as "transferring ownership" and it feels nice?).