changelog: add AI Memory and File Downloads

[Jocky-Teable]

Changes

• Added EN/ZH changelog entry for AI Memory and File Downloads
• Updated Mintlify banners for EN and ZH
• Added compressed MP4 demos for both feature sections

Preview

• http://localhost:3333/en/changelog#jul-15-2026
• http://192.168.0.80:3333/en/changelog#jul-15-2026

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