On the documentation, add StackBlitz example buttons for `Auth`

[SebConejo]

The aim is to give developers direct access to a demo on stackblitz showing how Auth works in a real use case.

• The stackblitz demo should present the backend.yml file as soon as the developer arrives.
• This demo includes a project with a backend manifest, and a front end that includes those authenticate features
  • Login
  • Signup
  • Get current user
  • Logout
• It should show the short and the long syntax.

Docs on relations: https://manifest.build/docs/authentication.

For the frontend, don't pull it with your own style code. Use a neutral library like shadcn/ui toolkit without applying extra css code.