Docs: add ml-diagnostic guide

[Shuang-cnt]

Description

MaxText has integrated the ML Diagnostics SDK in the code. This PR aims to introduce ML Diagnostic flag managed_mldiagnostics=True
This flag will be used with 3 different scenarios:

1. Only upload configs and metrics, not profiling (managed_mldiagnostics=True)
2. Upload configs and metrics, and do profiling on the first device (managed_mldiagnostics=True and profiler=xplane)
3. Upload configs and metrics, and do profiling on the all devices (managed_mldiagnostics=True, profiler=xplane, and upload_all_profiler_results=True)

Tests

I manually verified the updated instructions to ensure accuracy.

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.