Improve visibility of the simple robot

...ot_1dof/simple_robot_1dof/forward_kinematics_node_1dof.py

ros2_ws/src/simple_robot_1dof/urdf/simple_robot.urdf

ros2_ws/src/simple_robot_2dof/urdf/simple_robot.urdf

ros2_ws/src/simple_robot_3dof/urdf/simple_robot.urdf