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

tpl/tplimpl/embedded/templates/_default/_markup/render-image.html

@@ -1,7 +1,7 @@
 {{- $u := urls.Parse .Destination -}}
 {{- $src := $u.String -}}
 {{- if not $u.IsAbs -}}
-  {{- with or (.Page.Resources.Get $u.Path) (resources.Get $u.Path) -}}
+  {{- with or (.PageInner.Resources.Get $u.Path) (resources.Get $u.Path) -}}
     {{- $src = .RelPermalink -}}
   {{- end -}}
 {{- end -}}

tpl/tplimpl/embedded/templates/_default/_markup/render-link.html

@@ -1,11 +1,11 @@
 {{- $u := urls.Parse .Destination -}}
 {{- $href := $u.String -}}
 {{- if strings.HasPrefix $u.String "#" }}
-  {{- $href = printf "%s#%s" .Page.RelPermalink $u.Fragment }}
+  {{- $href = printf "%s#%s" .PageInner.RelPermalink $u.Fragment }}
 {{- else if not $u.IsAbs -}}
   {{- with or
-    ($.Page.GetPage $u.Path)
-    ($.Page.Resources.Get $u.Path)
+    ($.PageInner.GetPage $u.Path)
+    ($.PageInner.Resources.Get $u.Path)
     (resources.Get $u.Path)
   -}}
     {{- $href = .RelPermalink -}}