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/random_seed.h

@@ -22,6 +22,10 @@
 #define AVUTIL_RANDOM_SEED_H
 
 #include <stdint.h>
+/**
+ * @addtogroup lavu_crypto
+ * @{
+ */
 
 /**
  * Get random data.
@@ -33,4 +37,8 @@
  */
 uint32_t av_get_random_seed(void);
 
+/**
+ * @}
+ */
+
 #endif /* AVUTIL_RANDOM_SEED_H */