doc: Intro text for doxygen output in it's own file

(This patch has been modified to apply atop current master)

This makes it considerably easier to edit the text and make it different
for each library.

To address previous concerns with this patch, I wrote some more complete
introductory text. This is based on my understanding of these libraries, which
may not be correct, and is pretty rudimentary for libwayland-server!

However this intro text demonstrates how to create links to the
doxygen-generated text. It looks like you cannot link to methods easily as the
link name contains a hash number, but links to objects and classes work.
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Tested-by: Jon A. Cruz <jonc@osg.samsung.com>