fix: late catch of missing client args

[samdoghor]

This pull request introduces improvements to the developer onboarding experience and error handling in the SDK. The most significant changes are the addition of a comprehensive local setup guide, improved validation for client initialization, and updates to the test data file for clarity and usability.

Documentation improvements:

• Added a detailed local setup guide in DEVELOPMENT.md, covering prerequisites, installation steps, environment variable setup, example usage, troubleshooting, and project structure.

SDK usability and error handling:

• Updated TermiiClient initialization in termii_py/client.py to raise clear errors if api_key or base_url are missing, improving developer feedback and preventing silent failures.
• Revised test_data.py to demonstrate both environment variable-based and direct parameter initialization for TermiiClient, and clarified usage of the message sending API for easier testing and reference.