Document adding Vulcan into an existing Next apps

[eric-burel]

Is your feature request related to a problem? Please describe.
You have a Next app, you want to add a Vulcan backend in it
Describe the solution you'd like

What steps?

• Add the graphql api endpoint
• Add the auth system or at least the User model (that's the difficult part)
• Add TypeScript
• What else?

Describe alternatives you've considered
Take inspiration from Blitz recipe system for example?
In a first iteration, we could start by writing a doc about it.