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.
Rather than introducing "handle" to be the central term here I would probably write something like:  ...returns a reference to the allocated memory (you can think of a reference as a sort of handle by which you get access to the memory). IMHO, it's better not to introduce unnecessary terminology.