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

hugolib/shortcode.go

@@ -321,10 +321,16 @@ func prepareShortcode(
 
 	// Allow the caller to delay the rendering of the shortcode if needed.
 	var fn shortcodeRenderFunc = func(ctx context.Context) ([]byte, bool, error) {
+		if p.m.pageConfig.IsGoldmark && sc.doMarkup {
+			// Signal downwards that the content rendered will be
+			// parsed and rendered by Goldmark.
+			ctx = tpl.Context.IsInGoldmark.Set(ctx, true)
+		}
 		r, err := doRenderShortcode(ctx, level, s, tplVariants, sc, parent, p, isRenderString)
 		if err != nil {
 			return nil, false, toParseErr(err)
 		}
+
 		b, hasVariants, err := r.renderShortcode(ctx)
 		if err != nil {
 			return nil, false, toParseErr(err)