add Linux troubleshooting instructions

Author: tjhorner

docs/docs/01-build-guide/03-flash-and-configure.md

@@ -4,6 +4,10 @@ Next, we will flash the Transit Tracker firmware to the Matrix Portal board, con
 
 ## Flash Firmware
 
+:::note
+If you're flashing on a Linux machine, see [this section](../02-user-manual/03-troubleshooting.md#cant-flash-firmware-on-linux) regarding permissions if you are having trouble.
+:::
+
 1. Plug the board into your computer.
 2. Go to the <a href="/configurator" target="_blank">configurator</a> and press "Initial setup".
 3. Hold the "Boot" button on the board. While continuing to hold "Boot", press "Reset" at the same time, then let go of both buttons. The board is successfully in flashing mode if you don't see anything on the displays.

docs/docs/02-user-manual/03-troubleshooting.md

@@ -22,6 +22,20 @@ If you are having trouble pushing your Transit Tracker's configuration via USB,
 6. Try using a different USB port on your computer.
 7. If you're using a USB hub, try connecting directly to your computer instead.
 
+### Can't flash firmware on Linux
+
+If you are using a Linux machine and are having trouble flashing the firmware, you may need to give your user permission to access the USB device. For most distributions, you can follow these steps:
+
+1. Run the following command to add your user to the `dialout` group:
+   
+   ```bash
+   sudo usermod -aG dialout $USER
+   ```
+2. Log out and log back in for the group change to take effect.
+3. Try flashing the firmware again.
+
+If it's still not working, consult your distribution's documentation for more information on managing USB device permissions. If you are using something like Flatpak or Snap to run your browser, you may need to give it additional permissions to access USB devices.
+
 ## Status Messages
 
 If you see a status message on your Transit Tracker instead of the schedule, that can help diagnose an issue. Here are the possible status messages and their meanings.