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 the vein of concreteness, is it worth clarifying that it is a pointer? (e.g. "what we are given back is a handle to that memory (a pointer). This ...")

I guess some people might be confused calling it a handle without acknowledging the conventional term (i.e. be asking "is this handle somehow different to a pointer?? Is my life a lie?" etc. etc.)