docs: Connect 1,500+ apps to Teable AI

[Jocky-Teable]

Adds the September 21, 2026 announcement, “Connect 1,500+ apps to Teable AI,” in all 11 supported languages. Includes the approved introduction, shared cover image, and Gmail, Attio, and LinkedIn demos, plus localized help-center banners.

Also fills the missing September 18 and 20 changelog translations in nine languages. Existing entries are preserved. Includes the latest main update (d13b145) for connector event triggers and related documentation.

Validation: checked all 11 changelogs for entry order, historical content preservation, translated bullet counts, and valid MDX for the three latest entries. Checked rendered previews and media playback, and ran git diff --check.

Local preview: http://localhost:3480/en/changelog#sep-21-2026

[CLAassistant]

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