OAK-301: Document Oak

Remove oak-doc from the normal build, as it in any case needs to be
built separately with "mvn site", i.e. the default "mvn install" build
doesn't make much sense for oak-doc

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1562870 13f79535-47bb-0310-9956-ffa450edef68