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.
And perhaps another aside here to note that what's *really* happening is the implicit creation of an `int` in the data segment that's storing the value `5`, with `FOO` being a reference to *that*.