Update post-training docs to point to single source of truth for installation

[SurbhiJainUSC]

Description

Update installation instructions in post-training docs to point to single source of truth in install_maxtext.md

Tests

N/A - documentation update

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.