SFT documentation refine - single TPU VM procedure

[xibinliu]

Description

SFT documentation refine

Tests

Manually tested following the procedure.

Checklist

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

• 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.

[SurbhiJainUSC]

Please do the similar change for multi-hot guide too: https://github.com/AI-Hypercomputer/maxtext/blob/c670768d3965a632f0b609b3cc393e3462f05c1a/docs/tutorials/posttraining/sft_on_multi_host.md

[xibinliu]

Done!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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