Add headless bundle support

This commit adds  support for `headless bundles` for the `index` bundle type.

So:

```toml
headless = true
```

In front matter means that

* It will have no `Permalink` and no rendered HTML in /public
* It will not be part of `.Site.RegularPages` etc.

But you can get it by:

* `.Site.GetPage ...`

The use cases are many:

* Shared media galleries
* Reusable page content "snippets"
* ...

Fixes #4311

hugolib/hugo_sites.go

@@ -508,7 +508,11 @@ func (h *HugoSites) setupTranslations() {
 			shouldBuild := p.shouldBuild()
 			s.updateBuildStats(p)
 			if shouldBuild {
-				s.Pages = append(s.Pages, p)
+				if p.headless {
+					s.headlessPages = append(s.headlessPages, p)
+				} else {
+					s.Pages = append(s.Pages, p)
+				}
 			}
 		}
 	}
@@ -560,6 +564,10 @@ func (s *Site) preparePagesForRender(cfg *BuildCfg) {
 		pageChan <- p
 	}
 
+	for _, p := range s.headlessPages {
+		pageChan <- p
+	}
+
 	close(pageChan)
 
 	wg.Wait()