Update catnav.html

Added a direct link to the Github. Currently, you'd have to go to a category and then to a script, and then click on the View on Github button to get there..

Modified the Godot Engine link text, as it isn't just a 3D game engine; in fact, it's stronger as a 2D engine at the moment and could be used to make apps/tools in addition to games. Also the /wp in the URL is unnecessary and would break if the site is updated to move to the webroot or a different directory, so may as well just let the domain redirect as needed.