Improve NuGetContext documentation with JSON format requirements (#2175)

### ❔What, Why & How
Before this pull request, there was only a warning and guidance related
to the `GitHubPackagesContext`.

After encountering a misconfiguration of the `NuGetContext` during my
own setup, this change adds a corresponding warning and guidance for the
`NuGetContext`, specifically clarifying the required JSON format for
proper AL-Go functionality.

The goal is to ensure consistency across contexts and to reduce
configuration errors by making format requirements more explicit.