Modula-2: fix documentation layout

Modula-2: fix documentation layout

The Modula-2 documentation is rejected by older versions of Makeinfo because
the web of @node markers is fairly broken, apparently some subsections were
moved around, most notably between the Overview and Using sections, and the
@node markers were not (properly) adjusted.

gcc/
	* doc/gm2.texi (Overview): Fix @node markers.
	(Using): Likewise.  Remove subsections that were moved to Overview
	from the menu and move others around.