docs: document airflow extra in README and docs

[fderuiter]

💡 What: Added the airflow extra to the list of optional dependencies in README.md and updated docs/airflow.rst to use the pip install "imednet[airflow]" command instead of manual package installation.

🎯 Why: The airflow extra was defined in pyproject.toml but missing from the installation documentation. Users might not know it exists or might struggle with manual dependency management.

🧠 Cognitive Impact: simplifies the installation process for users wanting to use Airflow integration, making it consistent with other optional dependencies like pandas or sqlalchemy.

📖 Preview:
README.md:

pip install "imednet[airflow]"     # For Airflow integration

docs/airflow.rst:

pip install "imednet[airflow]"

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PR created automatically by Jules for task 14599139364719843688 started by @fderuiter

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