Add Security. Defender integration

[heblasco]

This pull request enhances the documentation and walkthrough for deploying and securing an AI chat application in Azure. The main focus is on integrating Microsoft Defender for Cloud into the deployment process and providing step-by-step instructions and resources for validating security. The changes clarify the security objectives and expand the learning resources to include Defender for Cloud guidance.

Security integration and validation:

• Updated the optional bonus challenge to explicitly include securing the AI chat application with Microsoft Defender for Cloud, and added security validation steps to the exercise goals and actions.
• Added a detailed walkthrough section with instructions for enabling Defender for Cloud, configuring AI service protection, setting up Guardrails and Content Safety, and validating security alerts in the web app.

Documentation and learning resources:

• Expanded the list of learning resources to include official Microsoft documentation for Defender for Cloud for both web apps and AI services.