chore: link to rspack-dev-guide and make content more general in CONTRIBUTING.md

[Boshen]

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Summary

🤖 Generated by Copilot at 6337f67

Rewrote the introduction of CONTRIBUTING.md to refer to the new development guide website and avoid repeating information. This change simplifies the contributing guide and makes it easier for potential contributors to find the relevant resources.

Walkthrough

🤖 Generated by Copilot at 6337f67

• Rewrite introduction paragraph of CONTRIBUTING.md to thank contributors and direct them to the development guide (link)

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Latest commit: 6337f67

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