Update launchpad documentation

See https://sling.apache.org/documentation/the-sling-engine/the-sling-launchpad.html , we should update the page to not refer to 8-SNAPSHOT after the release. Another item which needs checking is the 'Launchpad Bundles' section, which is out of date. We should replace it with a description of the features and the general start level assignments, and possibly add links to the (released) provisioning model.