Add tutorial for RL GRPO gemma4-e4b on v6e 4x8

[dasoto]

Description

Adding tutorial for RL GRPO gemma-4-e4b on v6e 4x8 cluster using xpk.

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rc/maxtext/checkpoint_conversion/to_huggingface.py	0.00%	2 Missing ⚠️

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