Add an "Asterisk Architecture Overview" section to the doxygen documentation.

This is a side project I've been poking at this week.  The intent is to discuss
Asterisk architecture in a top down fashion to help new developers understand how
Asterisk is put together.  There is a ton of stuff to write about, so this will
just continue to evolve over time.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@226606 65c4cc65-6c06-0410-ace0-fbb531ad65f3