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.
`num` is not mutable here:

~~~
<anon>:5:5: 5:14 error: cannot assign to immutable dereference of `Box` `*num`
<anon>:5     *num += 1;
             ^~~~~~~~~
~~~