add link to external tut

Author: PatrickFarley

articles/cognitive-services/form-recognizer/toc.yml

@@ -42,6 +42,8 @@
   items:
   - name: Use Form Recognizer with Logic Apps
     href: tutorial-form-recognizer-with-logic-apps.md
+  - name: Use Form Recognizer with Cognitive Search
+    href: https://docs.microsoft.com/azure/search/cognitive-search-custom-skill-form
 - name: How-to guides
   items:
   - name: Build a training data set