Clean up Sling information on projects.apache.org

The information at https://projects.apache.org/project.html?sling is outdated

• we have a different description now
• many links point to http
• only subversion repository is listed

The data seems to be generated from https://svn.apache.org/repos/asf/sling/site/doap/sling-doap.rdf and the projects site seems to be http://svn.apache.org/repos/asf/infrastructure/site-tools/trunk/projects/ .

I also found https://svn.apache.org/repos/asf/comdev/projects.apache.org/trunk/data/committees/sling.rdf, but not sure whether that is used at all.

See also https://projects.apache.org/guidelines.html