add latest milestone/release composite update sites

[dvojtise]

We already have an update site that point to the latest nightly build. (see #45)

It would be nice to have composite update sites for milestones and releases.

However, we need to make sure to point only to the latest version (see https://dev.eclipse.org/mhonarc/lists/cross-project-issues-dev/msg15408.html) instead of pointing to all published versions.