fix markdown docs linking issue (#40)

Author: xcao65

.github/workflows/pre-commit.yml

@@ -48,8 +48,9 @@ jobs:
       with:
         node-version: '12'
     - name: Install manual dependencies
+    # pin markdown-link-check version to support multi-level reference link
       run: |
-        sudo npm install -g markdown-link-check
+        sudo npm install -g markdown-link-check@3.8.7
         python -m pip install pre-commit
         pre-commit install
     - name: Run markdown checker

docs/Installation.md

@@ -74,7 +74,7 @@ You will need to clone the repository if you plan to modify or extend the
 ML-Agents Toolkit for your purposes. If you plan to contribute those changes
 back, make sure to clone the `main` branch (by omitting `--branch release_19`
 from the command above). See our
-[Contributions Guidelines](../com.unity.ml-agents/CONTRIBUTING.md) for more
+[Contributions Guidelines](CONTRIBUTING.md) for more
 information on contributing to the ML-Agents Toolkit.
 
 ### Install the `com.unity.ml-agents` Unity package

docs/Learning-Environment-Examples.md

@@ -18,7 +18,7 @@ This page only overviews the example environments we provide. To learn more on
 how to design and build your own environments see our
 [Making a New Learning Environment](Learning-Environment-Create-New.md) page. If
 you would like to contribute environments, please see our
-[contribution guidelines](../com.unity.ml-agents/CONTRIBUTING.md) page.
+[contribution guidelines](CONTRIBUTING.md) page.
 
 ## Basic
 

docs/Limitations.md

@@ -2,6 +2,6 @@
 
 See the package-specific Limitations pages:
 
-- [`com.unity.mlagents` Unity package](com.unity.ml-agents.md)
+- [`com.unity.mlagents` Unity package](https://docs.unity3d.com/Packages/com.unity.ml-agents@2.1/manual/index.html#known-limitations)
 - [`mlagents` Python package](ML-Agents-README.md)
-- [`mlagents_envs` Python package](ML-Agents-Envs-README.md)
+- [`mlagents_envs` Python package](../ml-agents-envs/README.md#limitations)

docs/Training-ML-Agents.md

@@ -509,7 +509,7 @@ Below is a list of the `sampler_type` values supported by the toolkit.
   - **parameters** - `intervals`
 
 The implementation of the samplers can be found in the
-[Samplers.cs file](../com.unity.ml-agents/Runtime/Sampler.cs).
+[Samplers.cs file](https://github.com/Unity-Technologies/ml-agents/blob/main/com.unity.ml-agents/Runtime/Sampler.cs).
 
 ##### Training with Environment Parameter Randomization