Add link to Google Machine Learning Crash Course to playground index.html

[vorburger]

as "back link", given that I've learnt about this (cool!) site from a link on https://developers.google.com/machine-learning/crash-course/reducing-loss/playground-exercise

[vorburger]

Hello, I'm doing a year end clean up of my personal https://github.com/pulls and wanted to ask if you would still consider merging this very old Pull Request, or if we should forget about and close this? (This is a bulk message I'm posting to many issues to gauge what is still revelant.)