improve(design): primary Dropdown.Button divider and background style

[dengfuping]

📦 Modified package

• @oceanbase/design
• @oceanbase/ui
• @oceanbase/icons
• @oceanbase/charts
• @oceanbase/util
• @oceanbase/codemod
• Other (about what?)

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• Component style update
• TypeScript definition update
• Bundle size optimization
• Performance optimization
• Enhancement feature
• Internationalization
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Workflow
• Other (about what?)

🔗 Related issue link

• None

💡 Background and solution

Before	After

📝 Changelog

Language	Changelog
🇺🇸 English	-
🇨🇳 Chinese	- 🐞 修复 Dropdown.Button 为主按钮时分割线超出和背景色不正确的问题。- 📖 新增 Dropdown 的文档和示例。

☑️ Self-Check before Merge

⚠️ Please check all items below before requesting a reviewing. ⚠️

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Tests is updated/provided or not needed
• Changelog is provided or not needed

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