examples/pup/motor_dc: Add battery box example.

doc/api/pupdevices/dcmotor.rst

@@ -26,6 +26,12 @@ Motors without Rotation Sensors
 Examples
 -------------------
 
+Making a train drive forever
+************************************
+
+.. literalinclude::
+    ../../../examples/pup/motor_dc/motor_dc_battery_box.py
+
 Making the motor move back and forth
 ************************************
 

examples/pup/motor_dc/motor_dc_battery_box.py

@@ -0,0 +1,13 @@
+from pybricks.pupdevices import DCMotor
+from pybricks.parameters import Port
+from pybricks.tools import wait
+
+# Initialize the motor.
+train_motor = DCMotor(Port.A)
+
+# Choose the "power" level for your train. Negative means reverse.
+train_motor.dc(50)
+
+# Keep doing nothing. The train just keeps going.
+while True:
+    wait(1000)