[Docs] Improve README: Structure, Formatting, and Visual Enhancements

[Ishita-Si]

Hi maintainers ,
I’m a contributor and would like to improve the README.md file to make it more structured, beginner-friendly, and visually appealing.

Planned Improvements:

• Rewrite the introduction for clarity and purpose
• Add a table of contents for easier navigation
• Standardize each algorithm’s section (e.g., clear complexity bullet points, code links)
• Replace/remove placeholders like “alt text”
• Fix minor typos (e.g., "dirst" → "first")
• Add relevant badges (Dart version, license, GitHub stats)
• Improve the Contribution section for first-time contributors
• Optionally add links to algorithm visualizations or sample code snippets