Add synthetic MNIST data example

[dlwh]

Summary

• update mnist_example.ipynb to generate synthetic data instead of downloading MNIST
• use optax for the optimizer and run one short training loop

Testing

• jupyter nbconvert --to notebook --execute docs/mnist_example.ipynb --inplace
• pytest -q

---

https://chatgpt.com/codex/tasks/task_e_68681a1d2ff48331b0964fb384719f71