Add individual porting efforts to the platforms directory. In addition to
IA-64 which was already there, Alpha, PPC, and SPARC were added. Also
update some other bits of the site to point to the new location.

Note: I didn't get a repo copy of alpha/alpha.sgml to

platforms/alpha.sgml because there wasn't any real useful history
to preserve.  I also left the alpha/ dir in place for now but added
an index.sgml to point to the new location.  I'll nuke the
directory once the translation teams have caught up with the
changes.

Suggested by: obrien