doxy: Disable the main index page.

Theres no usefull or even remotely complete information on it currently.
Which just leads to confusion.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/avutil.h

@@ -26,7 +26,7 @@
  * external API header
  */
 
-/**
+/*
  * @mainpage
  *
  * @section libav_intro Introduction