ATI/AMD GPU documentation moved

[Bridgman, John]

FYI, the amd.com web site has been updated to integrate ATI and AMD
technical materials, and as a result our GPU programming info is located
in a different place than before. 

There are at least two navigation paths that work (both start at
amd.com) :

Software Developers => Documentation & Articles => Developer Guides &
Manuals => Open GPU Documentation

ATI Products => Developers => Open GPU Documentation