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/rational.h

@@ -32,6 +32,11 @@
 #include <limits.h>
 #include "attributes.h"
 
+/**
+ * @addtogroup lavu_math
+ * @{
+ */
+
 /**
  * rational number numerator/denominator
  */
@@ -132,4 +137,8 @@ int av_nearer_q(AVRational q, AVRational q1, AVRational q2);
  */
 int av_find_nearest_q_idx(AVRational q, const AVRational* q_list);
 
+/**
+ * @}
+ */
+
 #endif /* AVUTIL_RATIONAL_H */