Claim Model Support for Qwen3-Next

[Rohan-Bierneni]

Description

This pr updates the Maxtext ReadMe with an announcement claiming model support for Qwen3-Next.

If the change fixes a bug or a Github issue, please include a link, e.g.,:
FIXES: b/448407748

Tests

To claim model support these tasks need to be completed:

Forward Pass Logit Check: https://paste.googleplex.com/5438633436905472
Sample Train Workload with stable loss: https://screenshot.googleplex.com/AKrwzS3wzfst7Gb
Inference via decode.py: https://paste.googleplex.com/5431522368815104
MMLU benchmark comparable to reported results: https://paste.googleplex.com/5408151231922176
Checkpoint Conversion script/util onboarding:

• Scanned script
• Unscanned script

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.

[parambole]

🔥

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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