yet another revision of documentation category stuff

Hello,

I've committed some more changes to the documentation
category stuff and posted the current html output here:
http://maxima.sourceforge.net/misc/tmp-categories-html/maxima_89.html

At this point it is close to finished in the sense that I can't
or won't think of something else to do. If there aren't too many
complaints I'll merge this stuff into the CVS trunk in a few days.

My plan at this point is to include the category stuff only in English-
language html output, not in .info or .pdf, and not in any translations.
When we're pretty sure the categories have stabilized we might spend
some time on translations.

Anyway I hope you find this useful.

Robert Dodier