Update documentation.

- Convert http to https, provide archive link when possible.
- Made CPU-JIT.png more readable on dark themes;
Added a white background so there isn't black text on a black
background.

xenia-build

@@ -304,7 +304,7 @@ def get_clang_format_binary():
         if has_bin(binary):
             return binary
     print('ERROR: clang-format is not on PATH')
-    print('LLVM is available from http://llvm.org/releases/download.html')
+    print('LLVM is available from https://llvm.org/releases/download.html')
     print('At least version 3.8 is required.')
     print('See docs/style_guide.md for instructions on how to get it.')
     sys.exit(1)