docs: Update toc.yml to move Material Toolkit Upgrading documentation at the correct level (#1251)

Author: agneszitte

doc/toc.yml

@@ -5,6 +5,9 @@
   # ***************** Material *******************
   - name: Material
     href: material-getting-started.md
+    items:
+    - name: Upgrading Material Toolkit
+      href: material-migration.md
   # ***************** Cupertino ******************
   - name: Cupertino
     href: cupertino-getting-started.md
@@ -78,5 +81,3 @@
   # ***************** Reference\Styles **********************
   - name: Styles
     href: controls-styles.md
-  - name: Upgrading Material Toolkit
-    href: material-migration.md