docs(airbox-q900): add QDL GitHub download link and Ubuntu udev rules for EDL verification

[tangzz-radxa]

Summary

Improve the Fogwise® AIRbox Q900 documentation for QDL tool download and Ubuntu EDL mode verification.

Changes

1. Download page (fogwise/airbox-q900/download.md, zh + en)

   • Add the Qualcomm GitHub repository (linux-msm/qdl) as an additional QDL tool download source, alongside the existing Qualcomm Software Center link.

2. Install system to onboard UFS (fogwise/airbox-q900/getting-started/install-system/onboard-ufs.md, zh + en)

   • Add a step-by-step udev rule setup in the "Verifying EDL Mode > Ubuntu" section before the existing lsusb check:
     • Create /etc/udev/rules.d/51-qcom-usb.rules with vendor/product IDs 05c6:9008
     • Reload udev rules and trigger device detection
     • Replug the USB cable
   • The existing lsusb verification steps remain unchanged after the new steps.

Background

• Ubuntu users without proper udev permissions may fail to see the Qualcomm QDL device (05c6:9008) with lsusb, which blocks the subsequent qdl flashing flow. Adding the udev rule steps reduces this common setup failure.
• QDL is also maintained on GitHub (linux-msm/qdl); linking it gives users an alternative official source.

Scope

• Docs only, 4 files (zh + en), single product scope (fogwise/airbox-q900).