This is a new feature : automatically generate HTML configuration guide (issue #2004)

This new doc generation is included in the build.xml  :
- a new target has been added : "docgen"
- this target is called at the same level as the javadoc target (i.e. from "all" and "nightly" targets)
- the doc is generated at : build/docgen/configuration_guide
(note that Brian is still working on the mockup, so the headers of the pages will have a nicer look)