PROTON-280: Added Engine documentation and configured Maven to generate HTML from it and from existing Messenger docs.
Moved existing docs down one level because Maven's doxia markdown plugin requires that markdown docs are in a markdown sub-folder.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1463950 13f79535-47bb-0310-9956-ffa450edef68
PROTON-260 : changes to reflect feedback from Rafi and Alan


git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1464126 13f79535-47bb-0310-9956-ffa450edef68