doc: style fixes for the TOC

- Hide the scrollbar on the TOC on all browsers. It was never the
  intention for it to be visible with the scroll indication in place.
  A wrapper element with 20px padding was added to accommodate for
  hopefully all scrollbar widths as well as to avoid overflowing
  content.
- Fixed the scroll indication gradient on Safari, which was caused by
  the wrong from-color, which now matches the to-color.
- Fixed a issue in old IE where the TOC didn't render on the correct
  position through setting `left: 0` and `top: 0` on it.

PR-URL: https://github.com/nodejs/node/pull/4748


Reviewed-By: James M Snell <jasnell@gmail.com>