Added index page and PCM page (partial documentation)

doc/index.doxygen

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+/*! \page Index Preamble and License
+
+\author Jaroslav Kysela <perex@suse.cz>
+\author Abramo Bagnara <abramo@alsa-project.org>
+\author Takashi Iwai <takashi@suse.de>
+\author Frank van de Pol <fvdpol@home.nl>
+
+<H2>Preface</H2>
+<P>The Advanced Linux Sound Architecture (\e ALSA) comes with a kernel
+API & library API. This document describes the library API and how
+it interfaces with the kernel API.</P>
+
+<H2>Documentation License</H2>
+
+<P>This documentation is free; you can redistribute it without
+any restrictions. The modification or derived work must retain
+copyright and list all authors.</P>
+ 
+<P>This documentation is distributed in the hope that it will be
+useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.</P>
+
+<H2>API usage</H2>
+<P>Application programmers should use the library API rather than
+kernel API. The library offers 100% of the functionally of the kernel API,
+but add major improvements in usability, making the application code simpler
+and better looking. In addition, some of the some fixes/compatibility code
+may be placed in the library code instead of the kernel driver.</P>
+
+<H2>API links</H2>
+
+<UL>
+  <LI>Page \ref pcm explains the design of PCM (digital audio) API
+</UL>
+
+*/