XWiki.OrphanedPages only supports the old parent reference and not nested pages

See:

• https://forum.xwiki.org/t/migrating-articles-create-orphaned-pages/10248
• https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-index/xwiki-platform-index-ui/src/main/resources/XWiki/OrphanedPages.xml
• In XWiki.LiveTableResults there's some hardcoded concept of orphaned (should be removed IMO)

  #if ("$!request.orphaned" == '1')
    #set ($homepage = $services.wiki.getById($services.wiki.currentWikiId).mainPageReference)
    #set ($homepageFullName = $services.model.serialize($homepage, 'local'))
    ## On Oracle the empty parent is actually null.
    #set ($extra = "${extra} and (doc.parent = '' or doc.parent is null) and doc.fullName <> :homepageFullName")
    #set ($discard = $params.put('homepageFullName', $homepageFullName))
  #end
  

TODO:

• Since we've moved to nested pages, the concept of orphaned pages doesn't really exist anymore. Orphaned pages are now top level pages.
• Listing top level pages doesn't seem very interesting but maybe it is?
• So we need to decide if we simply deprecate/remove this page (move it to a snippet on snippet.xwiki.org for example, creating an extension for it sounds too much) or if we want to list top level pages. IMO we don't, and we should rather focus on being able to express this in the LD filter for the Page index.