updated scribble CSS files for new docs layout
restore old Scribble default style, but make new style default for manuals

Using
 #lang scribble/base
produces HTML output in the old style, while
 #lang scribble/manual
uses the new style.

To get the new style without switching to `#lang scribble/manual`,
use `manual-doc-style`.

Related changes include the addition of `css-style-addition` and
`js-style-addition`.