update toc

Author: halkazwini

articles/network-watcher/toc.yml

@@ -167,6 +167,12 @@
       items:
       - name: Use Power BI
         href: network-watcher-visualize-nsg-flow-logs-power-bi.md
+      - name: Use Elastic Stack
+        href: /previous-versions/azure/network-watcher/network-watcher-visualize-nsg-flow-logs-open-source-tools/toc.json
+      - name: Use Grafana
+        href: /previous-versions/azure/network-watcher/network-watcher-nsg-grafana/toc.json
+      - name: Use Graylog
+        href: /previous-versions/azure/network-watcher/network-watcher-analyze-nsg-flow-logs-graylog./toc.json
   - name: Read flow logs
     href: flow-logs-read.md
   - name: Traffic analytics