docs: add Connect Everything Airtable migration guide

[garyli27]

Summary

• add an English and Chinese Airtable migration guide centered on AI Chat's Connect Everything skill
• make the guide prominent from Base, Import, and AI Chat documentation
• document validation, security, batching, and the built-in importer fallback

Validation

• PATH=/opt/homebrew/opt/node@20/bin:/opt/homebrew/bin:/opt/homebrew/sbin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/pkg/env/global/bin:/Users/guangyuli/.codex/tmp/arg0/codex-arg0pysjLN:/Users/guangyuli/.cache/codex-runtimes/codex-primary-runtime/dependencies/bin/override:/Users/guangyuli/.local/bin:/Users/guangyuli/.cache/codex-runtimes/codex-primary-runtime/dependencies/bin/fallback:/Applications/ChatGPT.app/Contents/Resources mintlify validate
• python3 -m json.tool docs.json
• git diff --check