Add comprehensive documentation for Demilitarized Zone (DMZ) in computer science

[Copilot]

This PR adds a comprehensive guide explaining the concept of Demilitarized Zone (DMZ) in computer science, addressing the question "What is de-militarisation zone in computer science?"

The new documentation (dmz-demilitarized-zone.md) covers:

Core Concepts:

• Definition and purpose of DMZ in networking and cybersecurity
• Security benefits including isolation, risk containment, and compliance
• Traditional three-tier, dual-homed, and screened subnet architectures

Practical Implementation:

• Common DMZ services (web servers, mail servers, DNS, VPN gateways)
• Security best practices with firewall configuration examples
• Network segmentation and access control strategies

Modern Applications:

# Example: AWS DMZ with CloudFormation
Resources:
  PublicSubnet:
    Type: AWS::EC2::Subnet
    Properties:
      CidrBlock: 10.0.1.0/24
      MapPublicIpOnLaunch: true

Advanced Topics:

• Cloud DMZ implementations (AWS, Azure, GCP)
• Container and Kubernetes DMZ with network policies
• Microservices DMZ using API gateways
• Monitoring, compliance (PCI DSS, NIST), and future trends

The documentation follows the established repository pattern with clear explanations, practical code examples, real-world use cases, and extensive references for further learning. It also updates the README.md to include the new topic under a "Network Security & Architecture" section.

Fixes #76.

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