Better approach to Scribble CSS.

- Add/use css/scribble.css. Note: The contents are more like
  racket.css in typical Scribbling -- for <span> elements like syntax
  highlighting. We don't use the whole page generated by Scribble, so
  block elements and things like table of contents are N/A.

- Don't add Pygments classes, too. Just go with the "RktXxx" Scribble
  classes.

- Do continue to add newlines (for the <pre> blocks), and may as well
  simplify some HTML like that for spaces as well as removing any
  empty RktMeta elements.