docs: add Stack Auth to list of auth libraries (#68284)

Adds Stack Auth to the list of auth libraries in the documentation.
Stack Auth is a managed, fully open-source authN & authZ platform
designed for Next.js.

Co-authored-by: Jiwon Choi <devjiwonchoi@gmail.com>

docs/02-app/01-building-your-application/09-authentication/index.mdx

@@ -1616,6 +1616,7 @@ Now that you've learned about authentication in Next.js, here are Next.js-compat
 - [Kinde](https://kinde.com/docs/developer-tools/nextjs-sdk)
 - [Lucia](https://lucia-auth.com/getting-started/nextjs-app)
 - [NextAuth.js](https://authjs.dev/getting-started/installation?framework=next.js)
+- [Stack Auth](https://docs.stack-auth.com/getting-started/setup)
 - [Supabase](https://supabase.com/docs/guides/getting-started/quickstarts/nextjs)
 - [Stytch](https://stytch.com/docs/guides/quickstarts/nextjs)
 - [WorkOS](https://workos.com/docs/user-management)