docs(setup): flash the microSD through the Pi with usbboot

[HuggeK]

What does this PR do?

Adds a tip to the Raspberry Pi setup guide. Raspberry Pi's usbboot (rpiboot) can make the Pi 5 appear to your computer as a USB drive, so you can flash its microSD card with Raspberry Pi Imager without a card reader.

Why was this required?

Many laptops no longer have an SD slot, and the setup guide assumed a card reader. The Pi can do the job itself over USB-C.

Automated tests

Not applicable: docs-only change, no code touched.

Manual (human) testing

Checked each step against the usbboot README's Pi 5 section and the Windows installer script (the shortcut name and the mass-storage-gadget command).

Checklist

• Single feature/fix — this PR is scoped to one thing with a clear story above
• Automated tests included — n/a, docs only
• Manual testing described — I documented what I verified by hand above
• Updated docs or CHANGELOG if needed
• No unrelated changes mixed in

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