feat: add linked list implementation in Python

[Poonam-Biradar]

🔗 Related Issue

Closes #14

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📝 What did you do?

Added a complete Singly Linked List implementation in Python.

Also created the folder structure DSA/LinkedList/ since it was not present in the repository.

Implemented the following operations:

• Insert at beginning
• Insert at end
• Delete node
• Search element
• Display linked list
• Reverse linked list

Added comments and example usage for better understanding.

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🗂️ Type of Change

• ✨ New solution / feature
• 🐛 Bug fix
• 📝 Documentation update
• 🧹 Code cleanup / refactor

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✅ Self-Review Checklist

• My code runs without errors
• I followed the folder structure from the README
• I added comments/docstrings explaining my logic
• My branch name is descriptive
• I linked the issue above
• I haven't modified unrelated files

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📸 Screenshots / Output (if applicable)

Example Output:

10 -> 20 -> 30 -> None
10 -> 30 -> None
30 -> 10 -> None

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💬 Anything else?

This contribution is beginner-friendly and can help new contributors understand basic Linked List operations in Python.