Add Documentation for Global Variables

[camilamaia]

Description:

The current documentation does not explain how Global Variables work in ScanAPI. This can create confusion for new users and limit the full use of the feature.

This issue aims to:

• Add a dedicated section about Global Variables to the official documentation.
• Provide practical usage examples in different contexts.
• Clarify the differences between global and local variables, including best practices.

Reference: Implemented in #265