Add comprehensive system tray menu examples to documentation

[s00d]

Description

I'm currently developing a cross-platform system tray application SwitchShuttle and have been extensively working with Tauri's system tray menu API. During development, I noticed that the current documentation could benefit from more practical examples showing real-world usage patterns.

This PR adds several comprehensive examples to the system tray documentation, including:

• Dynamic menu updates - How to update menu item text in real-time
• Background monitoring - Examples of updating menu items from background threads
• JavaScript integration - How to handle menu updates from the frontend
• Real-time data display - Practical examples of showing live system information in menu items

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