Add comprehensive guide on handling downstream system failures

[Copilot]

This PR addresses the learning topic "How do you handle the failure when downstream system is down?" by adding comprehensive educational content to the Dev-Journal repository.

Changes Made

New Documentation: downstream-failure-handling.md

Created a detailed guide covering essential patterns and strategies for handling downstream system failures in distributed systems, including:

• Circuit Breaker Pattern - Preventing cascading failures with proper state management
• Retry Mechanisms - Intelligent retry logic with exponential backoff and jitter
• Timeout Handling - Connection, read, and overall timeout strategies
• Graceful Degradation - Providing reduced functionality when services are unavailable
• Fallback Mechanisms - Alternative paths and cached responses
• Bulkhead Isolation - Resource isolation to prevent failure propagation
• Asynchronous Processing - Queue-based patterns for resilience

Additional Coverage

• Monitoring and observability best practices
• Implementation considerations and configuration management
• Testing strategies including chaos engineering
• Popular tools and libraries for different tech stacks
• Example architecture patterns and code snippets

Updated README

Enhanced the main README to include a "Topics Covered" section that references the new learning material, maintaining the repository's purpose as a development journal.

This content provides practical guidance for building resilient distributed systems and serves as a comprehensive reference for handling downstream service failures.

Fixes #4.

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