Mandatory documents are listed as pages that do not belong to extensions

Steps to reproduce: try to delete the XWiki space.

Since XWIKI-13954, a tree listing pages that are going to be deleted is display in case some of the concern pages belongs to a XAR.

However, mandatory documents are listed as "pages that do not belong to extension", which is not true (they belongs to the JAR extension) and they will be automatically recreated when the wiki will restart. So we should handle these pages differently (explaining they will be reinitialized instead of deleted) and reinitialize them on the fly instead of waiting for the next restart.