dox: Write a guide to the rust runtime
auto merge of #11501 : alexcrichton/rust/dox, r=brson

The official documentation sorely needs an explanation of the rust runtime and what it is exactly, and I want this guide to provide that information.

I'm unsure of whether I've been too light on some topics while too heavy on others. I also feel like a few things are still missing. As always, feedback is appreciated, especially about things you'd like to see written about!