Move the list of contributors to a dedicated XObject

Following https://forum.xwiki.org/t/automate-the-contributors-list-during-the-release/18663/, we agreed to extract the list of contributors to a dedicated XObject.

The main goal is to be able to add this list programmatically with a simple rest query to add the relevant XObject.

But this has more benefits like making it easier to get the list of all contributors for a full cycle more easily.