Improve .Content vs shortcodes

For the content from other pages in shortcodes there are some chicken and
egg dependencies that is hard to get around. But we can improve on this  by preparing the pages in a certain order:

 1. The headless bundles goes first. These are page typically page and image collections..
 2. Leaf bundles
 3. Regular single pages
 4. Branch bundles

Fixes #4632

hugolib/site.go

@@ -1865,14 +1865,15 @@ func getGoMaxProcs() int {
 
 func (s *Site) newNodePage(typ string, sections ...string) *Page {
 	p := &Page{
-		language: s.Language,
-		pageInit: &pageInit{},
-		Kind:     typ,
-		Source:   Source{File: &source.FileInfo{}},
-		Data:     make(map[string]interface{}),
-		Site:     &s.Info,
-		sections: sections,
-		s:        s}
+		bundleType: bundleBranch,
+		language:   s.Language,
+		pageInit:   &pageInit{},
+		Kind:       typ,
+		Source:     Source{File: &source.FileInfo{}},
+		Data:       make(map[string]interface{}),
+		Site:       &s.Info,
+		sections:   sections,
+		s:          s}
 
 	p.outputFormats = p.s.outputFormats[p.Kind]