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.
Another way you could phrase it that's less dismissive of the reader: "this often happens because the programmer's mental model of how ownership *should* work doesn't match the actual rules that Rust implements."  This could then lead into talking about how, as you gain experience with Rust, you start to internalise the rules.