[docs] Add links to example envs for Buffer Sensor and MultiAgentGroup (#5116)

* Update changelog with links to example envs

* Cooperative push block link update

com.unity.ml-agents/CHANGELOG.md

 ## [1.9.0-preview] - 2021-03-17
 ### Major Changes
 #### com.unity.ml-agents (C#)
-- The `BufferSensor` and `BufferSensorComponent` have been added. They allow the Agent to observe variable number of entities. (#4909)
+- The `BufferSensor` and `BufferSensorComponent` have been added. They allow the Agent to observe variable number of entities. For an example, see the [Sorter environment](https://github.com/Unity-Technologies/ml-agents/blob/release_15_docs/docs/Learning-Environment-Examples.md#sorter). (#4909)
-  end episodes in groups. (#4923)
+  end episodes in groups. For an example, see the [Cooperative Push Block environment](https://github.com/Unity-Technologies/ml-agents/blob/release_15_docs/docs/Learning-Environment-Examples.md#cooperative-push-block). (#4923)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - The MA-POCA trainer has been added. This is a new trainer that enables Agents to learn how to work together in groups. Configure
   `poca` as the trainer in the configuration YAML after instantiating a `SimpleMultiAgentGroup` to use this feature. (#5005)