changelog: Introducing Teable 3.0

[Jocky-Teable]

Summary

• add the Jul 23, 2026 Teable 3.0 changelog in English and Chinese
• embed the Teable 3.0 launch video
• add the Product Hunt product link to the English changelog
• update the English and Chinese docs banners

Validation

• validated docs.json with jq
• verified the Product Hunt URL and removed all placeholders
• ran git diff --check

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