Docs: Add missing debugging page to App Router docs (#69367)
No wording changes, just adding the missing "Debugging" page to the App
Router docs so that it can be indexed.
Feedback:
https://vercel.slack.com/archives/C06H9BEAUCB/p1724764339556119
Closes:
https://linear.app/vercel/issue/DOC-3281/add-missing-debugging-page-to-app-router-docs
