Chore: refactor directories + contribution guideline

[julio4]

Description

Added a CONTRIBUTOR.md file to provide guidelines for contributors.

Added an empty tests module with a TODO flag, related to #70.

Refactored and reorganized the structure of listings and book sources:

• Removed numeration from examples to facilitate easy movement of examples within the documentation.
• Separated the lib, tests, and example's contracts modules into separate files, reducing the reliance on mdbook anchors for improved clarity and maintainability.

This is a first step in improving the current book structure.

Checklist

• CI Verifier: Run cairo_programs_verifier successfully