Add exercises section

[tcaiazzi]

Summary

This PR introduces a new section called exercises to the repository. This section includes a series of Kathará exercises that can be automatically verified using the kathara-lab-checker tool.

Changes

• Added a new exercises directory containing Kathará-based exercises.
• Introduced two initial exercises:
  • 01-simple-configuration: A basic exercise designed to help users get familiar with Kathará and the process of running exercises with automatic validation.
  • 02-add-one-router: An exercise that involves adding a router and a host to a pre-configured network topology.

Documentation

• The README.md within the exercises section contains step-by-step instructions on how to install the kathara-lab-checker tool and prepare the environment to complete and validate the exercises.
• Each folder in this section represents a standalone exercise with its own configuration files, instructions, and
  expected output.