Update the Mantis Workflow document in doxygen.

(closes issue #17175)
Reported by: lmadsen
Patches:
      Bug_Tracker_Workflow.v2.txt uploaded by pabelanger (license 224)
Tested by: pabelanger, lmadsen

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@259438 65c4cc65-6c06-0410-ace0-fbb531ad65f3