Add the concept of Documentation URL

It would work as follows:

• Add a new xproperty for a documentation link reference
• If that property is filled, add a "Documentation" button next to the existing "Download", "Source", "Issues" buttons, and don't display the "Description" xproperty.

This is needed for the new xwiki.org doc reorg, see option 2 of https://forum.xwiki.org/t/xwiki-org-documentation-reorganization-batch-2/16966 for the context

Also make the Documentation the primary button and move the Download button to be last.

Last, add icons for all buttons for consistency.