Nango OAuth integration guide added

Background tasks often need to interact with third-party APIs on behalf of users, which typically requires building complex OAuth flows and managing token refresh logic. A new guide has been added detailing how to offload this entirely to Nango.

The documentation walks through authenticating API calls from inside a background task without handling tokens directly. Using a practical example of fetching open GitHub pull requests and summarizing them with Claude, the guide covers everything from the initial Next.js API route to the background task implementation. This provides a blueprint for integrating any of Nango's 250+ supported APIs, like Slack or Notion, into automated workflows.