docs: add opeartions in API docs (#557)

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Fixes #<issue_number_goes_here> 🦕

Author: ashleyxuu

docs/templates/toc.yml

@@ -60,6 +60,11 @@
       name: Indexers
     - name: pandas
       uid: bigframes.pandas
+    - items:
+      - name: Plotting
+        uid: bigframes.operations.plotting
+      - name: PlotAccessor
+        uid: bigframes.operations.plotting.PlotAccessor
     - items:
       - name: Series
         uid: bigframes.series.Series