Add external model bringup guide

[RissyRan]

Description

Create a community version for model bringup user guide, based on internal doc. b/471279823.

Please note, there is a TODO [scripts](TODO) depends on #3139

Tests

Manually check

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.

[github-actions]

🤖 Hi @RissyRan, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[github-actions]

📋 Review Summary

This pull request introduces an excellent and comprehensive guide for community contributors on bringing up new models in Maxtext. The document is well-structured, clear, and covers the entire workflow from architecture analysis to verification.

🔍 General Feedback

• The guide is very detailed and will be a great resource for the community.
• I've pointed out a few minor typos and a suggestion to update a link to point to the main branch instead of a specific commit.
• Overall, this is a great addition to the documentation.