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

[SebConejo]

The aim is to give developers direct access to a demo on stackblitz showing how relations 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 displays relations.
• It should show the short and the long syntax.

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

Just below this code, We could provide the button. So the developer could see the result. :

😃 User:
properties:
  - name

😺 Cat:
  properties:
    - name
  belongsTo:
    - User
    

Then, other stackblitz buttons can bring the user directly to the front-end that loads relations and filters them.

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