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/md5.h

@@ -23,6 +23,12 @@
 
 #include <stdint.h>
 
+/**
+ * @defgroup lavu_md5 MD5
+ * @ingroup lavu_crypto
+ * @{
+ */
+
 extern const int av_md5_size;
 
 struct AVMD5;
@@ -32,5 +38,9 @@ void av_md5_update(struct AVMD5 *ctx, const uint8_t *src, const int len);
 void av_md5_final(struct AVMD5 *ctx, uint8_t *dst);
 void av_md5_sum(uint8_t *dst, const uint8_t *src, const int len);
 
+/**
+ * @}
+ */
+
 #endif /* AVUTIL_MD5_H */