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: 
"the length of time that a reference is valid" may be better; since just (attempting to) keep a reference around longer does not extend it's lifetime. (I think saying "the scopes in which a reference is valid" might be even better, but I'm not sure.)