New ownership guide

This is the start of the new "Ownership" guide, to replace the lifetimes guide.

I'm still working on it, but I wanted to share my progress, as this guide is _very_ important, and I'd like as many eyes on it as possible. :smile: 
Ugh. Yeah. I don't know if that's the right way or not, it's awkward to only talk about 'handles' in this one place.