Add FastAPI example main.py witha simple GET endpoint

[risha-bit]

Added main.py as a simple FastAPI example demonstrating a basic GET endpoint.

The application defines a single route ("/") that returns a JSON message: { "message": "Hello, FastAPI!" }

This serves as a quick-start example for beginners to understand how to:

• Initialize a FastAPI app
• Define a GET endpoint
• Run it locally using Uvicorn

To run this example:

1. Install dependencies: pip install fastapi uvicorn

2. Run the app: uvicorn main:app --reload

3. Open in your browser: http://127.0.0.1:8000/