Add student prerequisites and de-emphasize the chapter key

[joelostblom]

Student feedback is that the chapter key was helpful, but not needed as a front and center piece in its own section. As we add a preface with information like student prereqs, we could include the chapter key at the bottom of that page instead with a test simialr to "you can either proceed to the next chapter or read the chapter overview below (you can always come back to this if something is unclear).

I think the preface should also clarify that while the narrative story is ficitonal, all the data is real and we hope students will learning interesting things about the world while learning how to visualize data in the best way possible.