Fix MaxText installation instructions in all jupyter notebooks and update doc

[SurbhiJainUSC]

Description

• Fix MaxText installation instructions in sft_qwen3_demo.ipynb, sft_llama3_demo_tpu.ipynb and rl_llama3_demo.ipynb
• Update run_python_notebook.md with the correct installation instructions
• Re-enabling jupyter notebook test disabled by Move RL code from src/MaxText/rl/ to src/maxtext/trainers/post_train/rl/ #3180

Tests

CI test

Checklist

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• 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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