Started update of DC walkthrough

I'm making the DC walkthrough more basic, showing only cf-sketch
interactive mode, and explaining more of the basic concepts at the
beginning. I've moved the original walkthrough, which I think still has
its place, to design-center-advaced.markdown, with the idea that it
contains the same steps, but showing the entire API interactions, for
more advanced users.

Not finished yet, more to come.
Fixed conflicts, added "More information" section.
Renamed advanced walkthrough.
Added more additional reference information.
Fixed formatting and links.