docs: add missing default value section to server.cors

[chenjiahan]

Summary

Add missing default value section to server.cors.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[Copilot]

Pull Request Overview

This PR updates the server.cors documentation to include a missing default value section, clarifying the configuration details for server.cors.

• Added a "Default" section in the server.cors documentation.
• Removed the existing "Type" line to instead focus on default value details.

Comments suppressed due to low confidence (1)

website/docs/en/config/server/cors.mdx:4

• The default section is introduced without specifying an actual default value. Please add the appropriate default value information to provide clarity to users.

+ **Default:**

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