docs: Add Quick Start Example to README

[YuChenLee1115]

Description

Added a Quick Start Example section to the README to help new users get started with MONAI more easily. The example demonstrates basic medical image loading and spacing adjustment.

Changes Made

• Added new "Quick Start Example" section to README
• Included code example for basic image loading
• Added example for image spacing adjustment
• Improved documentation readability

Purpose

This change aims to make MONAI more accessible to newcomers by providing a clear, practical example of basic functionality.

Testing Done

• Verified the markdown formatting
• Tested the code example locally
• Ensured proper integration with existing documentation