Nango OAuth integration guide being added to documentation

This PR adds a new documentation guide showing developers how to integrate Nango OAuth with Trigger.dev tasks. The guide walks through building a task that receives a Nango connection ID from a frontend, fetches a fresh GitHub access token from Nango, calls the GitHub API to retrieve open pull requests, and uses Claude to summarize them.

The integration works because Nango handles the complexities of OAuth — storing tokens, handling refreshes, managing 250+ provider integrations — so developers can focus on building their tasks rather than token management. Once a user connects their GitHub account through the Nango OAuth flow, their connection ID can be stored and reused across future task runs without re-authentication.

This lives in the frameworks section of the documentation, alongside guides for Next.js, Prisma, and Sequin.