docs(nnx): add native LoRA Gemma4 and Qwen3 notebooks and update tutorial guides

[RexBearIU]

Description

Adds native Gemma4 and Qwen3 (0.6B) LoRA and QLoRA tutorial notebooks for Flax NNX, along with accompanying post-training setup documentation. Also includes a follow-up fix for lora_llama3_demo.ipynb from PR #4708.

These tutorials cover both standard PEFT workflows:

1. Pathway A: Native LoRA Supervised Fine-Tuning (SFT) starting from converted Hugging Face checkpoints.
2. Pathway B: Native LoRA Pre-training & Checkpoint Restore with QLoRA.

Summary of Changes:

• src/maxtext/examples/gemma4_native_lora_demo.ipynb: Gemma4 Native LoRA and QLoRA demo notebook.
• src/maxtext/examples/qwen3_native_lora_demo.ipynb: Qwen3 Native LoRA and QLoRA demo notebook.
• src/maxtext/examples/lora_llama3_demo.ipynb: Follow-up fix from PR Fix JAX 0.11.0 sharding compatibility and SFT trainer signature for Tunix integration #4708 to set MODEL_NAME = "llama3.1-8b-Instruct", ensuring checkpoint conversion downloads the Instruct-aligned weights (meta-llama/Llama-3.1-8B-Instruct).
• docs/tutorials/posttraining/native_lora.md: Guide for native Flax NNX LoRA.
• docs/guides/run_python_notebook.md & docs/tutorials/post_training_index.md: Updated documentation indices.

Notice 1: Once all tests pass, the "pull ready" label will automatically be assigned.
This label is used for administrative purposes. Please do not add it manually.

Notice 2: For external contributions, our settings currently require an approval from a MaxText maintainer to trigger CI tests.

Tests

All tutorial notebooks (qwen3_native_lora_demo.ipynb, gemma4_native_lora_demo.ipynb, lora_llama3_demo.ipynb) have been extensively verified and executed cell-by-cell under standard TPU VM environments using papermill with zero errors (100% clean exit code 0).

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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[RexBearIU]

Rebased branch onto main and fixed Sphinx documentation build warnings regarding notebook links.

[SurbhiJainUSC]

Should be ${MODEL?}?

[RexBearIU]

Good catch, thank you. Updated model_name to use ${MODEL?} for consistency with the SFT command above.