Add a way to merge pages by language

As an example:

```html
{{ $pages := .Site.RegularPages | lang.Merge $frSite.RegularPages | lang.Merge $enSite.RegularPages }}
```

Will "fill in the gaps" in the current site with, from left to right, content from the French site, and lastly the English.

Fixes #4463

hugolib/page.go

@@ -263,6 +263,16 @@ type Page struct {
 	targetPathDescriptorPrototype *targetPathDescriptor
 }
 
+// Sites is a convenience method to get all the Hugo sites/languages configured.
+func (p *Page) Sites() SiteInfos {
+	infos := make(SiteInfos, len(p.s.owner.Sites))
+	for i, site := range p.s.owner.Sites {
+		infos[i] = &site.Info
+	}
+
+	return infos
+}
+
 // SearchKeywords implements the related.Document interface needed for fast page searches.
 func (p *Page) SearchKeywords(cfg related.IndexConfig) ([]related.Keyword, error) {