docs: recommend validated models by scenario

[EdisonSu768]

Summary

• Add scenario recommendations for chat, coding, and RAG under validated models.
• Use the Artificial Analysis open-weight leaderboard (weights=open) and document the Intelligence, Coding, and Agentic Index signals.
• Link the recommendation page from the validated models index and record deployment caveats for exact checkpoints, RAG evaluation, and deprecated MiniMax-M2.5.

Validation

• yarn lint
• yarn build
• git diff --check

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