[plt-scheme] New Help Desk

I've committed and exp-tagged files for the new PLT Help Desk.  The
files are located in the help collection.

The new Help Desk uses the PLT Web server and your favorite Web browser,
instead of relying on the PLT browser.

We're making this change because the PLT Web browser supports only a
limited subset of HTML, restricting what we can put into Help Desk.
Also, content on Web sites linked in Help Desk didn't always render well
in the PLT browser.

As before, Help Desk can be started with a launcher or from DrScheme.
The new launcher uses MzScheme, instead of MrEd, so you should delete
your old launcher.  Running Setup will build the new launcher.  

With the old Help Desk, you had to obtain new Help Desk content by
deleting plt/collects/doc/help/, and downloading a .plt file.  With the
new Help Desk, you just update the help collection, and run setup on
that collection.

WARNING: the "CVS refresh manuals" link on the manuals page doesn't work
properly yet.  There are some changes that need to be made to the Web
server.  I expect those changes will be in place within the next couple
of weeks.

Please let me know if you have any difficulties with this new code.

-- Paul