Posttraining documentation refine

[hsuan-lun-chiang]

Description

Improves post-training documentation for multimodal models and corrects file paths in the example commands.

Tests

Manually verified the documentation by running the commands for Checkpoint Conversion, Multimodal Decoding, and Supervised Fine-Tuning to ensure correctness.

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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