Simplify apidoc's RenderController::findRenderer to allow for custom renderers via ext

I could be missing something, but I changed apidoc's RenderController::findRenderer to do a simple class_exists check. That way, through Yii's autoloader, I can create an extension to load a custom template renderer. The previous method would not allow me to do this (I would have had to put my custom template inside a cloned version of yii2-apidoc).