78 errors in return value of models json example

[MaxenceGui]

• [x] Correct JSON errors in the example of nachet-model-documentation.md
• [ ] Modify labelOccurence to better reflect our data

[MaxenceGui]

Took the opportunity of this issue to update the documentation for Nachet backend

@ibrahim-kabir, @RussellJimmies and @Francois-Werbrouck if you want to give it a quick review I will appreciate it