docs: improve environment variables part

[OlivierBelaud]

This PR enhances our documentation on managing environment variables within GitHub workflows.

1. It clarifies where to locate these variables and secrets in GitHub repository settings, helping users verify NuxtHub’s synchronization.
2. For private repositories on free GitHub accounts or organizations, the documentation now explains that environment variables must be manually configured and provides clear instructions on where to do so.
3. An example workflow has also been updated to demonstrate the use of both a public variable and a secret for private variables.

[atinux]

Merci :)