chore: add the toc template file for entry page (#378)

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Fixes internal #325083413 🦕

Author: ashleyxuu

docs/templates/toc.yml

@@ -3,6 +3,16 @@
     name: Overview
   - href: changelog.md
     name: Changelog
+  - items:
+    - href: summary_overview.yml
+      name: Overview
+    - href: summary_class.yml
+      name: Classes
+    - href: summary_method.yml
+      name: Methods
+    - href: summary_property.yml
+      name: Properties and Attributes
+    name: BigQuery DataFrames API
   - items:
     - items:
       - name: Options