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.
I think the term "value" here is a little confusing, since there is one value and two references (pointers) to that value. Of course, a reference is also a value, but I think it would be better to say something like: ... Rust knows that's wrong. The `x` reference transferred ownership of the value to `num`. Hence the error: use of moved value: `x`.