docs: add Luma Uni-1 guide

[mintlify]

Adds a new "Luma Uni-1" tutorial under the Luma partner-node section in EN, ZH, and JA, summarizing Luma's official Uni-1 overview.

Highlights covered:

• Create vs Modify modes
• Core parameters (prompt, aspect ratio, seed, references)
• Reference image roles + example prompts
• Prompting guidelines and aspect ratios
• Seeds, advanced techniques, troubleshooting

Files:

• tutorials/partner-nodes/luma/luma-uni-1.mdx
• zh/tutorials/partner-nodes/luma/luma-uni-1.mdx
• ja/tutorials/partner-nodes/luma/luma-uni-1.mdx
• docs.json — added entries to all three Luma tutorial groups

[mintlify]

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Project	Status	Preview	Updated (UTC)
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[lin-bot23]

I noticed this PR is missing some content from the official launch blog post. I've prepared a follow-up commit that adds:

• Architecture context: Uni-1 is a decoder-only autoregressive transformer (not a diffusion model), reasons before generating
• Expanded features: text rendering, web search grounding, multilingual prompts, multi-panel output
• New 'Strengths' section covering key capabilities
• New 'Getting started' section with Cloud template links and Desktop instructions

Commit: lin-bot23@e1f3ccb

Branch: https://github.com/lin-bot23/docs/tree/mintlify/8de78c7f

This commit applies cleanly on top of the current PR branch (mintlify/8de78c7f). Happy to push it directly if you grant write access, or feel free to cherry-pick.

Also synced in ZH and JA translations.