Add Flax Examples and Cloud TPU README
# What does this PR do?

Adds a Flax examples README. Pretty bare for now, but will include a link to Cloud TPU instructions once they are up.

I hope my use of relative links works well, but looking for feedback. The main goal here is to have a canonical link we can point to. Perhaps later this should live on the proper docs page but I thought a README is a fine first step.

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## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

cc @patrickvonplaten 