Guide: add a pointer to play.rust-lang.org

As-is, there's no indication that the code examples pop out into a window that runs on `play.rust-lang.org` until you mouse over them. I managed to get to section 4 of the guide before realizing you could do this since it didn't occur to me to mouse over the example text.

cc @rose since we went through the tutorial together and I think it wasn't obvious to her either.