[tutorial] feat: add MoE notebook example

[karinseve]

What does this PR do ?

Add a self‑contained MoE (Mixture of Experts) training playbook

Changelog

Add examples/moe/ directory with a standalone MoE training example.

Provide a Jupyter notebook (moe.ipynb) walking through data setup, configuration, training, and basic evaluation for MoE models.

Include example configuration and launch commands tailored to Megatron‑Bridge MoE training.

Add brief inline documentation and comments to guide users adapting the playbook to their own setups.

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[yaoyu-33]

@karinseve thanks for adding a playbook. Can you move to tutorials/ folder since it's notebook.