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.
I wonder if it might help to briefly describe that even though parameters in Julia are given type annotations for dispatch, they can still be rebound / coerced to values of different types later. Then you can segue into your discussion of type stability.