Add Example Using Global Variables

[camilamaia]

The demo-api project does not currently showcase the usage of Global Variables in a real scenario. Adding an example will help users better understand how to apply this feature in practice.

This issue aims to:

• Create or update a specification in demo-api that uses Global Variables.
• Demonstrate a realistic use case (e.g., base URL, authentication token, or common headers).
• Ensure the example is clear, simple, and aligned with the ScanAPI documentation and tutorial.
• Link back to the related issues in the ScanAPI repository for consistency.

Reference: Feature implemented in ScanAPI #265