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__output.go

@@ -86,6 +86,7 @@ type pageOutput struct {
 	page.ContentProvider
 	page.PageRenderProvider
 	page.TableOfContentsProvider
+	page.RenderShortcodesProvider
 
 	// May be nil.
 	cp *pageContentOutput
@@ -99,6 +100,7 @@ func (p *pageOutput) initContentProvider(cp *pageContentOutput) {
 	p.ContentProvider = cp
 	p.PageRenderProvider = cp
 	p.TableOfContentsProvider = cp
+	p.RenderShortcodesProvider = cp
 	p.cp = cp
 
 }