Create comprehensive README.md with setup instructions and usage guide

[Copilot]

This PR adds a comprehensive README.md file to document the Azure OpenAI Console App with Entra ID authentication project. The documentation provides complete setup instructions and usage guidance for developers who want to use this solution.

What's included in the README:

Project Overview

• Clear description of the application's purpose and capabilities
• Key features including secure authentication, interactive chat, multimodal support, and content filtering

Prerequisites and Azure Setup

• Required software and Azure services
• Step-by-step instructions for creating Azure OpenAI resources
• Detailed Entra ID app registration process
• API permissions configuration
• Role assignment for Azure OpenAI access

Configuration Guide

• Two configuration methods: appsettings.json and environment variables
• Complete parameter reference table
• Example configurations with placeholder values

Installation and Usage

• Installation steps from cloning to running the application
• Usage examples for both text-only and image-based conversations
• Information about included sample images for testing

Technical Documentation

• Architecture overview with component descriptions
• Authentication flow explanation
• Dependencies and their purposes

Troubleshooting and Support

• Common issues and their solutions
• Debug tips for authentication and API problems
• Links to relevant Microsoft documentation

The README follows best practices with clear formatting, code examples, and comprehensive coverage of all aspects needed to successfully use the application.

Fixes #1.

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