Add a standalone demo program for ML-DSA and update the README.md file to include instructions to run the demo.

[keshavgarg5130]

Overview

This PR introduces a minimal, standalone demo program to demonstrate the basic usage of ML-DSA (mode 3) for keypair generation, message signing, and signature verification.

Changes

• Added a new demo/ folder at the root of the repository.
• demo/ contains:
  • demo.c — the example program.
  • Makefile — to compile and run the demo easily on Linux/macOS/Windows.
• The demo is isolated and does not modify any existing ML-DSA source files.
• Uses the library as intended through the public API (api.h).
• Updated the README.md file to include the instructions to run the demo program.

Motivation

• Provides a working example for beginners to quickly understand ML-DSA usage.
• Keeps the core repository untouched while adding educational value.
• Helps in testing the library without touching the main build system.

How to Build and Run

cd demo
make
./demo

Tested On

• macOS (Apple Silicon, arm64)

Notes

• No functionality of the main library is changed.
• The demo supports cross-platform compilation for Linux/macOS/Windows.