docs: ✍️ Scribe: Missing dotenv fallbacks in examples

[fderuiter]

💡 What: Added fallback shell export instructions (including IMEDNET_BASE_URL) directly alongside load_dotenv() usage in docs/quick_start.rst, docs/async_quick_start.rst, examples/quick_start.py, examples/async_quick_start.py, and examples/build_form_payload.py.
🎯 Why: dotenv.load_dotenv() fails silently if no .env file exists. If a user skips cp .env.example .env and there are no documented shell exports to fall back on, the application crashes with missing variable errors, frustrating first-time users.
🧠 Cognitive Impact: Fixes first-run crashes. Removes ambiguity about how to configure the application when not using a .env file, reducing the "Time to Understand" configuration requirements.
📖 Preview:

  # Option 1: Use a .env file (recommended)
  cp .env.example .env

  # Option 2: Export directly to your shell
  export IMEDNET_API_KEY="your_api_key"
  export IMEDNET_SECURITY_KEY="your_security_key"
  # Optional: Custom base URL for the API endpoint
  # export IMEDNET_BASE_URL="https://edc.prod.imednetapi.com"

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PR created automatically by Jules for task 16280606349452140816 started by @fderuiter

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