Add Global Variables Example to Tutorial

[camilamaia]

Description:

The tutorial currently does not cover how to use Global Variables in ScanAPI. Including this will make it easier for new users to understand and apply the feature in real scenarios.

This issue aims to:

• Update the tutorial with a clear example using Global Variables.
• Show how they can simplify configurations and reduce duplication.
• Highlight best practices for combining global and local variables.

Depends on: scanapi/demo-api#98
Reference: Implemented in #265