Add execution ipynb file and Google Colab link to run Gen on Google Colab on README.md.

[CanIyu]

Please check this movie https://drive.google.com/file/d/1BpjwA32iQBIlrTKYt99f4w0_lTwwkkP7/view?usp=sharing.
You can test that gen and julia enviroment quickly and easily on your own Google Colab within about 10min to run it.
You can try and run it until Example: Bayesian linear regression section of gen quick start.

[yawn-c111]

Updates

• Installation of dependencies, it can refer to packages needed Project.toml instead of hard-coding them.
  • So these installation will be done after switching to Julia.
• Changed google colab link button to refer to the original repository google colab file.

This change was made to avoid the need to rewrite the contents of each Colab notebook every time the dependencies of gen-quickstart are updated to match the Gen updates. The types and versions of installed packages are usually found in the same Project.toml file referenced by the Jupyter notebook tutorials.