docs: add Connect Everything migration hub

[garyli27]

Summary

• move Connect & Migrate Everything under the AI documentation
• add dedicated Airtable, Baserow, SmartSuite, NocoDB, and More Sources pages in English and Chinese
• keep each migration page concise and source-specific
• redirect the previous Base migration URLs to the new AI hub and Airtable page

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