DOXYGEN: Add doxy groups to remaining headers in 'graphics'

To make sure that they appear in the graphics group
in the doc structure.

graphics/renderer.h

@@ -28,6 +28,15 @@
 
 namespace Graphics {
 
+/**
+ * @defgroup graphics_renderer Renderer
+ * @ingroup graphics
+ *
+ * @brief API for managing renderer engines.
+ *
+ * @{
+ */
+
 /**
  * List of renderer types
  *
@@ -56,7 +65,7 @@ Common::String getRendererTypeCode(RendererType type);
 
 /** Get the best matching renderer among available renderers */
 RendererType getBestMatchingAvailableRendererType(RendererType desired);
-
+ /** @} */
 } // End of namespace Graphics
 
 #endif