scribblings config: 'keep-style and 'no-search flag, 'omit-start section

The 'keep-style avoids imposing the Racket manual style on a
document, while 'no-search omits the search box. The 'omit-start
section is like 'omit, but the document is still indexed and
available for cross-references.

Also, recognize a 'prefixable tag shape so that the set of tags
hat support prefixing is extensible.
scribble: document all `scribble/base` CSS style classes

A demo document (linked from the Scribble manual) demonstrates how
to trigger each style class and what it looks like with the default
and manual styles.