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

Point them to their equivalent in https://skia.org.

BUG=None
R=hcm@google.com, jcgregorio@google.com
NOTRY=true

Review URL: https://codereview.chromium.org/856353004
diff --git a/Makefile b/Makefile
index 68e7b29..baaabfa 100644
--- a/Makefile
+++ b/Makefile
@@ -25,7 +25,7 @@
 # If you want more fine-grained control, you can run gyp and then build the
 # gyp-generated projects yourself.
 #
-# See https://sites.google.com/site/skiadocs/ for complete documentation.
+# See https://skia.org for complete documentation.
 
 SKIA_OUT ?= out
 BUILDTYPE ?= Debug
@@ -67,7 +67,7 @@
 
 uname := $(shell uname)
 ifneq (,$(findstring CYGWIN, $(uname)))
-  $(error Cannot build using Make on Windows. See https://sites.google.com/site/skiadocs/user-documentation/quick-start-guides/windows)
+  $(error Cannot build using Make on Windows. See https://skia.org/user/quick/windows)
 endif
 
 # If user requests "make all", chain to our explicitly-declared "everything"