Add API authentication, project setup, and Bronze data collection structure

This merge request includes:

• Project base structure (src/, data/, notebooks/)
• Environment configuration (.env.example, .gitignore)
• Added API authentication and token retrieval using environment variables.
• RTE API authentication module (api_rte.py)
• Updated README documentation
• Implemented connection to RTE “actual_generations_per_production_type” endpoint.
• Automated monthly data collection starting from 01/01/2020.
• Organized raw JSON data by year, month, and day under data/bronze/.
• Verified that all available historical data (2020–present) is correctly downloaded and stored.