Fix relative links in the Changelog. (#3911)

* Fix relative links in the Changelog.

* fix link references.

* fix link references.

* Inline links.

com.unity.ml-agents/CHANGELOG.md

   - Introduced the `SideChannelManager` to register, unregister and access side
     channels.
   - `Academy.FloatProperties` was replaced by `Academy.EnvironmentParameters`.
-    See the [Migration Guide](../docs/Migrating.md) for more details on
-    upgrading.
+    See the [Migration Guide](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Migrating.md)
+    for more details on upgrading.
   - `SideChannel.OnMessageReceived` is now a protected method (was public)
   - SideChannel IncomingMessages methods now take an optional default argument,
     which is used when trying to read more data than the message contains.
   now happens by default, and use `--resume` to resume training instead. (#3705)
 - The Jupyter notebooks have been removed from the repository. (#3704)
 - Removed the multi-agent gym option from the gym wrapper. For multi-agent
-  scenarios, use the [Low Level Python API](../docs/Python-API.md). (#3681)
+  scenarios, use the [Low Level Python API](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Python-API.md). (#3681)
-  [Low Level Python API documentation](../docs/Python-API.md) for more
-  information. If you use `mlagents-learn` for training, this should be a
+  [Low Level Python API](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Python-API.md)
+  documentation for more information. If you use `mlagents-learn` for training, this should be a
   transparent change. (#3681)
 - Added ability to start training (initialize model weights) from a previous run
   ID. (#3710)