Adding links to the Google Colab notebook tutorials

[vincentpierre]

Proposed change(s)

Adding 3 tutorial colab notebooks. They use the registry to access prebuilt environments.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

Since the notebooks are not on github, it will be hard to test, but the code is fixed on a version of ml-agents.