Add Page.RenderShortcodes

A layouts/shortcodes/include.html shortcode may look like this:

```html
{{ $p := site.GetPage (.Get 0) }}
{{ $p.RenderShortcodes }}
```

Fixes #7297

hugolib/page_test.go

@@ -1998,7 +1998,6 @@ func TestRenderWithoutArgument(t *testing.T) {
 		IntegrationTestConfig{
 			T:           t,
 			TxtarString: files,
-			Running:     true,
 		},
 	).BuildE()