Update CONTRIBUTING.md to improve readability

[Vicky-8223]

I reorganized the Apache Beam contributing guide into a cleaner, more readable Markdown format with:
1.Better structure - Clear sections with logical flow
2.Improved formatting - Proper headings, code blocks, and spacing
3.Enhanced navigation - Added Quick Reference section
4.All content preserved - No information lost

[gemini-code-assist]

Summary of Changes

Hello @Vicky-8223, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly refactors the CONTRIBUTING.md file to enhance its readability and usability. The primary goal is to provide a more structured and visually appealing guide for contributors, ensuring that all necessary information is easily discoverable and digestible. The changes focus on improving the presentation of existing content rather than introducing new policies or procedures.

Highlights

• Improved Readability: The CONTRIBUTING.md file has been reorganized into a cleaner, more readable Markdown format.
• Enhanced Structure: The guide now features clear sections with a logical flow, making it easier to navigate.
• Better Formatting: Proper headings, code blocks, and spacing have been applied throughout the document for visual clarity.
• New Quick Reference Section: A new 'Quick Reference' section has been added to provide easy access to important links and commands.
• Content Preservation: All original content and information from the previous version of the contributing guide have been retained.

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[github-actions]

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R: @Abacn added as fallback since no labels match configuration

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[github-actions]

Reminder, please take a look at this pr: @Abacn