[gtk/matthiasc/for-main] docs: Add a cross-reference

[Matthias Clasen <matthiasc src gnome org>]

commit 9f8abc5d744cb735a0e5b4a96c69774b06ff5193
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Oct 12 22:25:35 2022 -0400

    docs: Add a cross-reference
    
    Note that GtkTreeView is deprecated and link to the
    list view introduction from 'Tree and List Widget Overview'.

 docs/reference/gtk/section-tree-widget.md | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/docs/reference/gtk/section-tree-widget.md b/docs/reference/gtk/section-tree-widget.md
index 2fcf467ba1..17d3b51482 100644
--- a/docs/reference/gtk/section-tree-widget.md
+++ b/docs/reference/gtk/section-tree-widget.md
@@ -1,6 +1,13 @@
 Title: Tree and List Widget Overview
 Slug: gtk-treeview
 
+This document describes the `GtkTreeView` widget and auxiliary
+classes, like tree models and cell renderers. All of these have
+been deprecated and will be removed in GTK 5. Their replacements
+are described in the [List Widget Overview](section-list-widget.html).
+
+## Introduction
+
 To create a tree or list in GTK, use the `GtkTreeModel` interface in
 conjunction with the `GtkTreeView` widget. This widget is designed around
 a _Model/View/Controller_ design and consists of four major parts: