[Docs] Correct Resend cookbook setup

[roomote-community]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• update the product-updates cookbook recipe to use Roomote's built-in Resend integration
• replace the obsolete custom MCP server and API-key setup with the current deployment-scoped OAuth flow under Settings > Integrations

Why this change was made

A comprehensive public-documentation audit found that this recipe still described Resend's retired setup path. The correction matches the current integration registry and Resend guide without duplicating task and widget documentation already covered by draft PR #2615.

Impact

Operators following the recipe now receive the supported Resend setup instructions. No product behavior changes.

Screenshots

The local Mintlify preview lost generated Next.js files after hot reload, so this is a disclosed representative rendering on the local docs browser surface. It proves the corrected wording's appearance and readability, not genuine Mintlify navigation or runtime behavior.

[roomote-community]

No code issues found. See task

_{Reviewed 5969cc2}