[docs] Update changelog to release_3_docs links (#4098)

Author: Ervin T

com.unity.ml-agents/CHANGELOG.md

@@ -14,15 +14,15 @@ and this project adheres to
 - `max_step` in the `TerminalStep` and `TerminalSteps` objects was renamed `interrupted`.
 - `beta` and `epsilon` in `PPO` are no longer decayed by default but follow the same schedule as learning rate. (#3940)
 - `get_behavior_names()` and `get_behavior_spec()` on UnityEnvironment were replaced by the `behavior_specs` property. (#3946)
-- The first version of the Unity Environment Registry (Experimental) has been released. More information [here](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Unity-Environment-Registry.md)(#3967)
+- The first version of the Unity Environment Registry (Experimental) has been released. More information [here](https://github.com/Unity-Technologies/ml-agents/blob/release_3_docs/docs/Unity-Environment-Registry.md)(#3967)
 - `use_visual` and `allow_multiple_visual_obs` in the `UnityToGymWrapper` constructor
 were replaced by `allow_multiple_obs` which allows one or more visual observations and
 vector observations to be used simultaneously. (#3981) Thank you @shakenes !
 - Curriculum and Parameter Randomization configurations have been merged
   into the main training configuration file. Note that this means training
   configuration files are now environment-specific. (#3791)
 - The format for trainer configuration has changed, and the "default" behavior has been deprecated.
-  See the [Migration Guide](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Migrating.md) for more details. (#3936)
+  See the [Migration Guide](https://github.com/Unity-Technologies/ml-agents/blob/release_3_docs/docs/Migrating.md) for more details. (#3936)
 - Training artifacts (trained models, summaries) are now found in the `results/`
   directory. (#3829)
 - When using Curriculum, the current lesson will resume if training is quit and resumed. As such,
@@ -83,7 +83,7 @@ vector observations to be used simultaneously. (#3981) Thank you @shakenes !
   - Introduced the `SideChannelManager` to register, unregister and access side
     channels. (#3807)
   - `Academy.FloatProperties` was replaced by `Academy.EnvironmentParameters`.
-    See the [Migration Guide](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Migrating.md)
+    See the [Migration Guide](https://github.com/Unity-Technologies/ml-agents/blob/release_1_docs/docs/Migrating.md)
     for more details on upgrading. (#3807)
   - `SideChannel.OnMessageReceived` is now a protected method (was public)
   - SideChannel IncomingMessages methods now take an optional default argument,
@@ -112,9 +112,9 @@ vector observations to be used simultaneously. (#3981) Thank you @shakenes !
   `--load`. (#3705)
 - The Jupyter notebooks have been removed from the repository. (#3704)
 - The multi-agent gym option was removed from the gym wrapper. For multi-agent
-  scenarios, use the [Low Level Python API](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Python-API.md). (#3681)
+  scenarios, use the [Low Level Python API](https://github.com/Unity-Technologies/ml-agents/blob/release_1_docs/docs/Python-API.md). (#3681)
 - The low level Python API has changed. You can look at the document
-  [Low Level Python API](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Python-API.md)
+  [Low Level Python API](https://github.com/Unity-Technologies/ml-agents/blob/release_1_docs/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