Add SageMaker Studio Lab link to navigation on LTB pages

[Amazon SageMaker Studio Lab](https://studiolab.sagemaker.aws/) is a free service that developer can use JupyterLab on the browser. It will be beneficial to provide the link for opening the tutorial by Studio Lab.

By referring the past pull request, we think that we should modify the `layout.html` and `intro.py`.
https://github.com/pytorch/tutorials/pull/1541
