Add docstrings to level-00 exercise files

[travisjneuman]

Description

The Level 00 exercise files would benefit from module-level docstrings that briefly describe what the exercise teaches. This helps learners who open the file and want quick context.

Acceptance Criteria

• Add a one-line module docstring to each exercise.py in projects/level-00-absolute-beginner/
• Docstrings should describe the learning objective (e.g., """Learn to store values in variables and print them.""")
• Do not change any existing code logic

[travisjneuman]

Resolved in 36dceb1: Added module docstrings to all 15 level-00 exercise.py files describing learning objectives.