Maven Configuration for development and authentication

A couple a clean up needs to be performed on the Maven usage description:

• The following paragraph has to be removed, I tried it but I have errors (the artefact does not seem to be available anymore: https://doc.nuxeo.com/corg/maven-usage/#generate-a-new-project-with-the-nuxeo-archetype-start-archetype
• I am not sure we still need that piece, JDK6 at some point might not be our target anymore: https://doc.nuxeo.com/corg/maven-usage/#fix-maven-eclipse-plugin-warning-about-workspaceand39s-vm-mac-os-x-only 
• Updating the settings.xml is necessary, the old maven repository is not up to date
  • http://maven.nuxeo.org -> packages.nuxeo.com
  • This modification needs because the credentials have changed (require github token)

See for infos:

https://github.com/nuxeo/nuxeo/commit/48857f16cd9dfee19a8b2eeb3457799033666390