docs: GPT-6 launch and complete September 5 translations

[Jocky-Teable]

Summary

• Add the September 7 GPT-6 announcement with the approved cover and emphasis in all 11 supported languages.
• Complete the September 5 Query Deduplication and AI Import Optimization translations in the 9 locales that were missing them.
• Update localized documentation announcement bars to link to GPT-6. Existing changelog entries are preserved unchanged.

Base Verification

• Fetched origin immediately before preparing this PR.
• Confirmed GitHub main and local baseline both resolve to cde19dc.

Validation

• git diff --check passed.
• Validated docs.json, balanced Update tags, 11 GPT-6 locales, and 9 added September 5 translations.
• Verified that every pre-existing changelog entry is preserved byte-for-byte.
• Local previews were provided for review.

PR only. Do not merge or deploy automatically.

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