Docs: Improve clarity and structure of contribution guide

[inland-taipen]

Hello Specter team!

As a new contributor looking to get involved, I was reading the CONTRIBUTING.md file. I noticed all the important information was there, but it could be restructured to make it a bit easier for a newcomer to read and find what they need.

This PR reformats the entire file into a clear, well-structured guide.

Summary of Changes:

• Added a Table of Contents for easy navigation.
• Created a clear, step-by-step How to Contribute section.
• Added a Development Setup section with quick-start commands and a link to the more detailed docs.
• Organized existing information into clear, top-level sections:
  • Code Standards
  • Testing (with subsections for Backend and Frontend)
  • Finding Help
  • Contributors and Maintainers
• Added a friendly "Thank you" message at the end.

This PR does not change any code. It is purely a documentation improvement intended to help new contributors (like me!) get started more smoothly.

Looking forward to your feedback!

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