docs: Add comprehensive README documentation with usage guide

[girishlade111]

Summary

This PR enhances the README.md file with a comprehensive, professionally structured introduction and usage guide tailored specifically for the system-design-primer repository.

Changes Made

Minor Non-Breaking Change

• Added a comment marker for version tracking: <!-- Minor update: Enhanced README documentation for better clarity and usability -->

Major Documentation Enhancements

1. 📋 Table of Contents - Added structured navigation with links to all major sections

2. 🎯 About This Repository - Comprehensive introduction explaining:

   • What the repository is and who it's for
   • Key features including comprehensive coverage, interview-readiness, multi-language support, etc.

3. 👥 Who Should Use This Guide - Clear target audience breakdown:

   • Software Engineers & Developers
   • Students & Recent Graduates
   • Technical Leaders
   • Career Switchers

4. 📖 How to Use This Repository - Structured learning paths:

   • 2-4 week interview preparation guide
   • 8-12 week comprehensive learning plan
   • Daily study routine recommendations

5. 🎓 Learning Path - Progressive skill development:

   • Beginner Level concepts and projects
   • Intermediate Level topics
   • Advanced Level systems

6. Enhanced Anki Flashcards Section:

   • Step-by-step usage instructions
   • Benefits of spaced repetition

7. 🤝 Expanded Contributing Section:

   • Ways to contribute
   • Contribution guidelines

8. 📜 License & 🙏 Acknowledgments - Added proper attribution and licensing information

Impact

• User Experience: Significantly improved for new users and returning contributors
• Discoverability: Better structured content makes it easier to find relevant information
• Onboarding: Clear learning paths help users at different skill levels
• Backward Compatibility: All changes are additive; no existing content was removed or broken

Testing

• ✅ All internal anchor links verified
• ✅ Markdown formatting validated
• ✅ No breaking changes to existing structure

Checklist

• Changes are non-breaking
• README maintains existing style and tone
• All links are functional
• Content is accurate and helpful
• Follows repository's contributing guidelines