Pass .RenderShortcodes' Page to render hooks as .PageInner

The main use case for this is to resolve links and resources (e.g. images) relative to the included `Page`.

A typical `include` would similar to this:

```handlebars
{{ with site.GetPage (.Get 0) }}
  {{ .RenderShortcodes }}
{{ end }}
```

And when used in a Markdown file:

```markdown
{{% include "/posts/p1" %}}
```

Any render hook triggered while rendering `/posts/p1` will get `/posts/p1` when calling `.PageInner`.

Note that

* This is only relevant for shortcodes included with `{{%` that calls `.RenderShortcodes`.
* `.PageInner` is available in all render hooks that, before this commit, received `.Page`.
* `.PageInner` will fall back to the value of `.Page` if not relevant and will always have a value.

Fixes #12356

resources/page/pagemeta/page_frontmatter.go

@@ -88,6 +88,9 @@ type PageConfig struct {
 
 	// User defined params.
 	Params maps.Params
+
+	// Compiled values.
+	IsGoldmark bool `json:"-"`
 }
 
 // FrontMatterHandler maps front matter into Page fields and .Params.