docs: Update CONTRIBUTING.md to clarify documentation requirements for exported types

[zyfy29]

I mentioned it in the #3666 but forgot to do in the former pr. I think it's better to notify the rule here as well.

[Copilot]

Pull Request Overview

This PR updates the CONTRIBUTING.md documentation to clarify that documentation requirements apply to both exported methods and types, not just methods. It adds an example showing how to properly document exported types following Go documentation conventions.

• Expands documentation requirements from "exported methods" to "exported methods and types"
• Adds a concrete example demonstrating proper type documentation format

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[Copilot]

The phrase 'returned type' is misleading since Repository may not necessarily be a return type in all contexts. Consider using 'exported type' instead to be more accurate and consistent with the updated guideline.

Suggested change

	And the returned type `Repository` will have comments like this:
	And the exported type `Repository` will have comments like this: