docs(notification): improve system notification docs

[cc-hearts]

🤔 This is a ...

• 📝 Site / documentation improvement
• 📽️ Demo improvement

🔗 Related Issues

N/A

💡 Background and Solution

This PR improves the Notification component documentation and demos to better explain system-level notification behavior, permission requirements, and notification management limitations.

It also updates the demos with English copy, clearer button labels, static method usage, auto-close examples, and tag-based close examples.

📝 Change Log

Language	Changelog
🇺🇸 English	Improve Notification documentation and demos with permission notes, system settings guidance, FAQ, and clearer examples.
🇨🇳 Chinese	完善 Notification 文档和示例，补充权限说明、系统设置指引、FAQ，并优化示例展示。