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
