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/page__content.go

@@ -522,6 +522,7 @@ func (c *cachedContent) contentRendered(ctx context.Context, cp *pageContentOutp
 			if err != nil {
 				return nil, err
 			}
+
 			if !ok {
 				return nil, errors.New("invalid state: astDoc is set but RenderContent returned false")
 			}
@@ -626,8 +627,10 @@ func (c *cachedContent) contentToC(ctx context.Context, cp *pageContentOutput) (
 			return nil, err
 		}
 
-		// Callback called from above (e.g. in .RenderString)
+		// Callback called from below (e.g. in .RenderString)
 		ctxCallback := func(cp2 *pageContentOutput, ct2 contentTableOfContents) {
+			cp.otherOutputs[cp2.po.p.pid] = cp2
+
 			// Merge content placeholders
 			for k, v := range ct2.contentPlaceholders {
 				ct.contentPlaceholders[k] = v