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

markup/goldmark/internal/render/context.go

@@ -41,6 +41,7 @@ func (b *BufWriter) Flush() error {
 type Context struct {
 	*BufWriter
 	positions []int
+	pids      []uint64
 	ContextData
 }
 
@@ -55,6 +56,30 @@ func (ctx *Context) PopPos() int {
 	return p
 }
 
+// PushPid pushes a new page ID to the stack.
+func (ctx *Context) PushPid(pid uint64) {
+	ctx.pids = append(ctx.pids, pid)
+}
+
+// PeekPid returns the current page ID without removing it from the stack.
+func (ctx *Context) PeekPid() uint64 {
+	if len(ctx.pids) == 0 {
+		return 0
+	}
+	return ctx.pids[len(ctx.pids)-1]
+}
+
+// PopPid pops the last page ID from the stack.
+func (ctx *Context) PopPid() uint64 {
+	if len(ctx.pids) == 0 {
+		return 0
+	}
+	i := len(ctx.pids) - 1
+	p := ctx.pids[i]
+	ctx.pids = ctx.pids[:i]
+	return p
+}
+
 type ContextData interface {
 	RenderContext() converter.RenderContext
 	DocumentContext() converter.DocumentContext