Add new article for the cluster bubble layer.

Author: stevemunk

articles/azure-maps/toc.yml

@@ -367,6 +367,8 @@ items:
       href: power-bi-visual-understanding-layers.md
     - name: Add a bubble layer
       href: power-bi-visual-add-bubble-layer.md
+    - name: Add a cluster bubble layer
+      href: power-bi-visual-cluster-bubbles.md
     - name: Add a pie chart layer
       href: power-bi-visual-add-pie-chart-layer.md
     - name: Add a 3D column layer