[MLton] web site improvements

[Stephen Weeks]

I spent the better part of the day learning enough PHP to use it for
the MLton web site.  I redesigned things a bit, and it is now much
easier to make changes while keeping all the pages in synch.

Here are the main changes.

Added a search button in the same place on every page.  This uses
Google to search the whole site.  I removed the mailing-list specific
searches on the home page.  It doesn't seem like a good idea to cut
out relevant search results from other places, e.g. the user guide,
that may help the searcher answer their question.

Added a documentation page, and moved the history and papers pages
there.

Added a little bit of second level navigation information on pages
that aren't at the top level (like the history page).

Hacked the user guide HTML so it shows the same header as our other
pages.


Let me know of any problems or suggestions for further improvement.