Add module documentation support

Add the ability for modules to be documented using a /doc subpage, which
is automatically transcluded onto the module page.

To get the transcluding to work right, I wound up having to change from
the deprecated-in-1.21 ArticleViewCustom hook to ContentHandler, as
there didn't seem to be any other way to get the ParserOutput into the
links tables. Which means Scribunto now needs MediaWiki 1.21 rather
than 1.20.

Change-Id: Id487097c2a505c11f92a3404f5d3ee98beb2570c