Update TOC with unity-overview.md

Author: mattwojo

articles/spatial-anchors/toc.yml

@@ -10,6 +10,8 @@
   - name: Create Unity app
     expanded: true
     items:
+    - name: Unity overview
+      href: unity-overview.md
     - name: HoloLens
       href: quickstarts/get-started-unity-hololens.md
     - name: Android