Remove most of the contents of the doc dir in favor of the wiki content.

Remove most of the contents of the doc dir in favor of the wiki content.

This merge does the following things:

 * Removes most of the contents from the doc/ directory in favor
   of the wiki - http://wiki.asterisk.org/

 * Updates the build_tools/prep_tarball script to know how to export
   the contents of the wiki in both PDF and plain text formats so that
   the documentation is still included in Asterisk release tarballs.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@294740 65c4cc65-6c06-0410-ace0-fbb531ad65f3