feat: Update documentation and add new quick start test script

[fderuiter]

This pull request improves the documentation and code clarity for the iMednet Python SDK, focusing on clearer usage instructions, enhanced documentation structure, and improved code readability through docstrings. The most significant changes are grouped below.

Documentation Improvements:

• Added a prominent note in README.md and docs/index.rst clarifying that the SDK is unofficial, and linked to full documentation for easier access. [1] [2]
• Updated the quick start and async usage examples in both README.md and documentation files to use the simplified import pattern (from imednet import ImednetSDK, load_config), and added cross-references between synchronous and asynchronous usage. [1] [2] [3] [4]
• Expanded the documentation build and contribution instructions, including a step to validate documentation builds cleanly and a pointer to documentation guidelines.
• Improved the environment variable documentation by converting the list in docs/configuration.rst into a table for better readability.
• Expanded docs/cli.rst to document global CLI options and behaviors, including .env loading and help/version flags.

Code Quality and Clarity:

• Added descriptive docstrings to all major data models in imednet/models/records.py, imednet/models/sites.py, imednet/models/studies.py, and imednet/models/subjects.py to clarify their purpose and usage. [1] [2] [3] [4] [5] [6] [7] [8]

Testing and Examples:

• Added a new scripts/quick_start_test.py script to demonstrate SDK initialization and provide a basic test for setup verification.