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.
I don't know who specifically you want to target with this guide.  I feel as though Rust could end up being "my first systems language" for people coming from dynamic languages (like Python), as it's a whole lot shinier and (in some ways) more approachable than C/C++.  It **might** be worth briefly touching on GC and RC from the very rough perspective of "Python/Java manage all this for you behind the scenes, but there are performance and memory costs, which make it less than ideal for a system's language."

This is also not necessarily the right spot for such discussion, if added.