Add performance examples and caching strategies to CI caching documentation

Overview

This issue adds comprehensive performance examples and real-world caching strategies to the CI caching documentation. The additions include a performance comparison table showing build time improvements across different project types, before-and-after code examples for Node.js, Python, and Docker projects, and multi-project cache key strategies. These enhancements help users understand the tangible benefits of caching and implement it effectively.

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What needs to be done

• Add "Performance Examples: Real-World Build Time Improvements" section with comparison table showing time savings across different project types
• Include "Before and After Examples" section with practical YAML configurations for Node.js, Python, and Docker projects
• Add "Multi-Project Cache Key Strategies" section with project-specific cache key patterns
• Ensure all YAML examples are properly formatted and syntactically correct
• Verify table formatting renders correctly in markdown
• Ensure the new sections integrate smoothly with existing content
• Validate that all cache key strategies follow GitLab CI/CD best practices

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Files to be changed

• doc/ci/caching/_index.md

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Labels

~"group::documentation" ~"technical writing" ~"size::2"