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/base64.h

@@ -23,6 +23,13 @@
 
 #include <stdint.h>
 
+/**
+ * @defgroup lavu_base64 Base64
+ * @ingroup lavu_crypto
+ * @{
+ */
+
+
 /**
  * Decode a base64-encoded string.
  *
@@ -51,4 +58,8 @@ char *av_base64_encode(char *out, int out_size, const uint8_t *in, int in_size);
  */
 #define AV_BASE64_SIZE(x)  (((x)+2) / 3 * 4 + 1)
 
+ /**
+  * @}
+  */
+
 #endif /* AVUTIL_BASE64_H */