MAHOUT-1450 Cleaning up clustering documentation

git-svn-id: https://svn.apache.org/repos/asf/mahout/trunk@1587003 13f79535-47bb-0310-9956-ffa450edef68
MAHOUT-1483: Organize links in web site navigation bar

git-svn-id: https://svn.apache.org/repos/asf/mahout/trunk@1587081 13f79535-47bb-0310-9956-ffa450edef68
Merge remote-tracking branch 'remotes/origin/trunk' into trunk