Add checkpoint conversion instructions to GRPO docs

[khatwanimohit]

Description

• add environment variable section
• add checkpoint conversion instructions

If the change fixes a bug or a Github issue, please include a link, e.g.,: FIXES: b/123456 FIXES: #123456

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Tests

n/a

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• [x] I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• [x] I have necessary comments in my code, particularly in hard-to-understand areas.
• [x] I have run end-to-end tests tests and provided workload links above if applicable.
• [x] 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.

[github-actions[bot]]

🤖 Hi @khatwanimohit, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.