Add comprehensive firewall documentation to Dev-Journal

[Copilot]

This PR addresses the firewall learning question by creating comprehensive documentation that covers both aspects asked in the issue:

1. What is the use of a firewall? - Added detailed explanation covering 5 main categories:

   • Access Control
   • Traffic Monitoring and Filtering
   • Threat Prevention
   • Network Segmentation
   • Compliance and Logging

2. Are there physical and software firewalls? - Provided thorough comparison including:

   • Hardware/Physical firewalls with characteristics, examples, and use cases
   • Software firewalls (both host-based and network-based)
   • Hybrid approaches (virtual firewalls, NGFWs)
   • Decision criteria for choosing between types

The documentation includes practical examples, real-world use cases, and best practices to serve as a comprehensive reference for the Dev-Journal knowledge base. Also updated the README.md to include navigation to the new firewall documentation.

This follows the repository's purpose as stated: "write my notes in comments and increase the knowledge bank" by providing detailed technical content that can be referenced for learning.

Fixes #20.

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