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/error.h

@@ -27,6 +27,13 @@
 #include <errno.h>
 #include "avutil.h"
 
+/**
+ * @addtogroup lavu_error
+ *
+ * @{
+ */
+
+
 /* error handling */
 #if EDOM > 0
 #define AVERROR(e) (-(e))   ///< Returns a negative error code from a POSIX error code, to return from library functions.
@@ -65,4 +72,8 @@
  */
 int av_strerror(int errnum, char *errbuf, size_t errbuf_size);
 
+/**
+ * @}
+ */
+
 #endif /* AVUTIL_ERROR_H */