[Doc] Update Clerk guide to Next.js app router

[ymc9]

https://zenstack.dev/docs/guides/authentication/clerk

[andrictham]

@ymc9 I managed to get Clerk working with ZenStack and App Router

Here's what my code looks like in /src/app/api/model/[...path]/route.ts: https://gist.github.com/andrictham/1f4d729f33a63ab1fa817b40418f6e35

[ymc9]

@ymc9 I managed to get Clerk working with ZenStack and App Router

Here's what my code looks like in /src/app/api/model/[...path]/route.ts: https://gist.github.com/andrictham/1f4d729f33a63ab1fa817b40418f6e35

Cool! Great to know that! I'll incorporate it into the docs.

Thank you for the help!