docs: generalize Connect Everything migration guide

[garyli27]

Summary

• reposition Connect Everything as a general migration skill rather than an Airtable-only workflow
• cover Airtable, Baserow, NocoDB, SmartSuite, and other API-accessible systems
• simplify the guide to four steps and preserve the requested Airtable example
• redirect the previous Airtable-specific URLs to the new canonical guide

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