Docs: Improve README to describe parallel execution

[adrien-berchet]

No description provided.

[Copilot]

Pull Request Overview

This PR enhances the README documentation by adding comprehensive information about parallel execution capabilities in the dir-content-diff library. The changes restructure existing sections with better organization and add detailed guidance on performance optimization through concurrent file comparisons.

• Reorganizes existing documentation sections with descriptive subheadings for better navigation
• Adds extensive documentation for parallel execution features including thread-based and process-based concurrency options
• Provides practical usage examples and performance considerations for different execution modes

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