added a new item in the toc_template.j2 for the local build MD file

Author: AnsMelanie

doc/source/operators_doc/operator-specifications/toc_template.j2

@@ -18,6 +18,8 @@
     href: user-guide/using-operators.md
   - name: Workflow examples for beginners
     href: user-guide/workflow-examples.md
+  - name: Build local documentation
+    href: user-guide/dpf-docs-local-preview.md
 - name: Core concepts
   items:
   - name: Available types