Create Colab Snippets

I find myself often copying setup code (e.g. for a simple MNIST-like example) when trying out new features.

It would be very handy to have specially crafted Snippets for Flax development for this purpose. We could provide these snippets as a normal Colab notebook that lives on Github and they would automatically get updated when we change the source. Users imply add them to their config as described in e.g. [so/48760503](https://stackoverflow.com/questions/48760503)