added a list of available usage examples (#216)

Author: kyuan-mongodb

source/usage-examples.txt

@@ -71,3 +71,16 @@ usage example. If your instance uses :manual:`SCRAM authentication
 address or URL of your instance. Consult the
 :doc:`Connection Guide </fundamentals/connection>` for more information
 about getting connected to your MongoDB instance.
+
+Available Usage Examples
+------------------------
+
+- :doc:`Find Operations </usage-examples/find-operations>`
+- :doc:`Insert Operations </usage-examples/insert-operations>`
+- :doc:`Update Operations </usage-examples/update-and-replace-operations>`
+- :doc:`Delete Operations </usage-examples/delete-operations>`
+- :doc:`Count Documents </usage-examples/count>`
+- :doc:`Retrieve Distinct Values of a Field </usage-examples/distinct>`
+- :doc:`Run a Command </usage-examples/command>`
+- :doc:`Watch for Changes </usage-examples/changeStream>`
+- :doc:`Perform Bulk Operations </usage-examples/bulkWrite>`