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.
Yes, you definitely need to decide if you're targeting people already familiar with C++-style resource management, or folks with only GC language experience.

If the latter, it might be a good idea to start out with some other kind of resource (e.g. file handles) that everyone would be familar with, to show the universal acquire/use/release pattern, and then go on to note that in Rust this applies to memory as well.