Add the graphics codex to resources (#64)

* Add the graphics codex to resources

* Update index.html

Author: ShadowMitia

resources/index.html

@@ -83,6 +83,7 @@
 		* [Game Math](https://gamemath.com/)
 		* [A Comparison of Modern Graphics APIs](https://alain.xyz/blog/comparison-of-modern-graphics-apis)
 		* [Scratchapixel](https://www.scratchapixel.com)
+		* [The Graphics Codex](https://graphicscodex.com/)
 
 	</code>