Adding Text to Static Analysis Chapter

I've added the beginning (intro sections) of the chapter. I'm going to add 4 more sections, one for each of three "checks" that TypeCheck can make.

There probably needs to be more "this is how this works in Julia" notes added to the early code examples.
Not something you asked for feedback on, but I think adding a bit to the intro about how these sorts of checks seem "mystical" to most programmers might be good. e.g. when your compiler or tools in your favourite language does these things for you, it may seem like magic -- we're going to show you how it works.