Updated installation instructions. Added installation instructions to doc home page. (#5989)

Author: miguelalonsojr

docs/Installation.md

@@ -82,7 +82,7 @@ release. Omitting that will get the `main` branch which is potentially unstable.
 
 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_20`
+back, make sure to clone the `develop` branch (by omitting `--branch release_20`
 from the command above). See our
 [Contributions Guidelines](../com.unity.ml-agents/CONTRIBUTING.md) for more
 information on contributing to the ML-Agents Toolkit.

mkdocs.yml

@@ -8,6 +8,7 @@ copyright: com.unity.ml-agents copyright © 2017 - 2022 Unity Technologies
 nav:
 - Home: index.md
 - ML-Agents Overview: ML-Agents-Overview.md
+- Installation: Installation.md
 - Toolkit Documentation: ML-Agents-Toolkit-Documentation.md
 - Background:
     - Machine Learning: Background-Machine-Learning.md