d: Add language reference section to documentation files.

Adds an initial body of documentation for the D front-end - other than
the existing documentation for command-line usage/the man page.

Documentation covers code generation choices specific to GNU D - what
attributes are supported, intrinsics, pragmas, predefined versions,
language extensions, missing features and deviations from spec.

More could be added or elaborated upon, such as what linkage do
different symbols get, mixed language programming with C and C++, the
anatomy of a TypeInfo and ModuleInfo object, and so on.  This is enough
as a first wave just to get it off the ground.

gcc/d/ChangeLog:

	* Make-lang.in (D_TEXI_FILES): Add d/implement-d.texi.
	* gdc.texi: Adjust introduction, include implement-d.texi.
	* implement-d.texi: New file.