[PXCT-1549] Uno Q app as default instructions

[91volt]

Summary

This PR updates the UNO Q User Manual to include instructions on how to configure an application to run automatically at startup within the Arduino App Lab environment.

What This PR Changes

• Added "Running an App at Startup" section: A new guide inserted after the "Hello World Example" section.
• Documented UI Workflow: Explained how to use the dropdown menu next to the "Run" button to toggle the startup feature.
• Clarified Example Limitations: Added a note explicitly stating that built-in examples must be copied/duplicated before they can be set as the startup app.
• Added CLI Method: Included the arduino-app-cli command for advanced users who prefer the terminal.
• Added Visual Aid: Uploaded assets/run-at-startup.png to illustrate the location of the toggle switch.

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