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.
This could possibly do with at least mentioning the word "reference" and/or the symbol `&` since they're the things Rust uses for borrowing?