doxy: provide a start page and document libavutil

Introduce a basic layout, the subpages are currently left empty.

Split libavutil in multiple groups as example of the structure

libavutil/imgutils.h

@@ -22,6 +22,9 @@
 /**
  * @file
  * misc image utilities
+ *
+ * @addtogroup lavu_picture
+ * @{
  */
 
 #include "avutil.h"
@@ -127,4 +130,9 @@ int av_image_check_size(unsigned int w, unsigned int h, int log_offset, void *lo
 
 int ff_set_systematic_pal2(uint32_t pal[256], enum PixelFormat pix_fmt);
 
+/**
+ * @}
+ */
+
+
 #endif /* AVUTIL_IMGUTILS_H */