Added Flatten Binary Tree to Linked List

[VanshGarg06]

Issue number if any:-

Fixes #408
Kindly merge this PR with labels and into the main as i have checked everything is correct. Under the necessary labels @GSSOC2025 and level number as per issue #408

✅ Contribution Checklist (make sure all are checked)

• Code file added (.cpp, .java, .py, etc.)
• Markdown file added with:
  • Problem statement
  • Example input/output
  • Generic logic
  • Time & space complexity

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📝 Type of Addition (check any one)

• DSA problem addition
• Fix mistakes in existing codebase
• Add better solutions
• Make docs better

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📊 Last check (make sure all are checked

• Have you checked that this problem isn't already in the codebase?🔍
• Did you cross-check that your code/explanation is right?

🏢 Your Organization?

• GsSOC'25

[github-actions]

Thanks for creating a PR for your Issue! ☺️

We'll review it as soon as possible.
In the meantime, please double-check the file changes and ensure that all commits are accurate.

If there are any unresolved review comments, feel free to resolve them. 🙌🏼

[VanshGarg06]

Please Merge this PR with labels.

[github-actions]

🎉 Congrats on getting your PR merged!🙌🏼

Thanks for the contribution — every small chunk of contribution helps improve the project.
Looking forward to your next contribution! 🥳✨