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.
Missing an i in ownership in the start of the sentence "Rust's ownershp system ...". Otherwise, great guide, keep up the good work!