[MLton] wiki -> documentation

[Matthew Fluet]

>> Would it be worth adding the 'class = "external"' attribute to wiki-links
>> that point back to mlton.org?  I'm thinking of Download.  There are also
>> all of the "ViewSVN" links.
>
> From looking at the CSS, I don't see that "external" does anything.
> But perhaps the desired effect is to get the little globe before links
> to mlton.org.  I tweaked grab-wiki to do that, and committed a new
> src/doc/guide.

Yes, it was the globe image before links.  I suppose we could have our own 
different image (like the "ML" icon for mlton.org), but something to 
denote going out to the web.

In any case, the globe is fine.