Fix references to https://sites.google.com/site/skiadocs/.

skia.gyp

 # Top-level gyp configuration for Skia.
 #
 # Projects that use Skia should depend on one or more of the targets
 # defined here.
 #
 # More targets are defined within the gyp/ directory, but those are
 # not intended for external use and may change without notice.
 #
-# Full documentation at https://sites.google.com/site/skiadocs/
+# Full documentation at https://skia.org.
 #
 {
   'targets': [
     {
       'target_name': 'alltargets',
       'type': 'none',
       'dependencies': [
         'gyp/everything.gyp:everything',
         'gyp/most.gyp:most',
       ],
     },
   ],
 }