Add download site descriptor for old sanselan downloads (missed that in the last commit)

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/imaging/trunk@1512905 13f79535-47bb-0310-9956-ffa450edef68
Shamelessly steal index page layout from [lang]

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/imaging/trunk@1512906 13f79535-47bb-0310-9956-ffa450edef68