document graph traversal reimplementation for developers

Documents the changes to server code brought by https://github.com/openmicroscopy/openmicroscopy/pull/3228; some of the links may remain broken until it is merged. Mostly staged at http://www.openmicroscopy.org/site/support/omero5.1-staging/developers/Server/ObjectGraphs.html.
--no-rebase
@mtbc : I wonder if this should be joined. Will there be separate pages for Chgrp, etc.?

@hflynn : ok to continue the migration of items under `/modules/` to `/Server` etc?