Add documentation for custom tf_prefix

ur_simulation_gz/doc/custom_prefix.rst

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+Using a custom tf_prefix
+========================
+
+In case you would like to use a custom prefix to your simulated robot, you can provide one by
+passing the ``tf_prefix`` to the control launchfile:
+
+.. code-block:: bash
+
+   ros2 launch ur_simulation_gz ur_sim_control.launch.py tf_prefix:=my_ur_
+
+However, when doing so, you will have to provide your own controller configuration file. A
+corresponding file to the prefix above could look as follows:
+
+.. literalinclude:: resources/ur_controllers.yaml
+
+Using the ``$(var tf_prefix) notation as in the driver does not work here, since the file is loaded
+differently, where (to our knowledge) no variable substitution is possible.

ur_simulation_gz/doc/resources/ur_controllers.yaml

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+controller_manager:
+  ros__parameters:
+    update_rate: 500  # Hz
+
+    joint_state_broadcaster:
+      type: joint_state_broadcaster/JointStateBroadcaster
+
+    force_torque_sensor_broadcaster:
+      type: ur_controllers/ForceTorqueStateBroadcaster
+
+    joint_trajectory_controller:
+      type: joint_trajectory_controller/JointTrajectoryController
+
+    forward_velocity_controller:
+      type: velocity_controllers/JointGroupVelocityController
+
+    forward_position_controller:
+      type: position_controllers/JointGroupPositionController
+
+
+speed_scaling_state_broadcaster:
+  ros__parameters:
+    state_publish_rate: 100.0
+    tf_prefix: "my_ur_"
+
+
+force_torque_sensor_broadcaster:
+  ros__parameters:
+    sensor_name: my_ur_tcp_fts_sensor
+    state_interface_names:
+      - force.x
+      - force.y
+      - force.z
+      - torque.x
+      - torque.y
+      - torque.z
+    frame_id: tool0
+    topic_name: ft_data
+
+
+joint_trajectory_controller:
+  ros__parameters:
+    joints:
+      - my_ur_shoulder_pan_joint
+      - my_ur_shoulder_lift_joint
+      - my_ur_elbow_joint
+      - my_ur_wrist_1_joint
+      - my_ur_wrist_2_joint
+      - my_ur_wrist_3_joint
+    command_interfaces:
+      - position
+    state_interfaces:
+      - position
+      - velocity
+    state_publish_rate: 100.0
+    action_monitor_rate: 20.0
+    allow_partial_joints_goal: false
+    constraints:
+      stopped_velocity_tolerance: 0.2
+      goal_time: 0.0
+      my_ur_shoulder_pan_joint: {trajectory: 0.2, goal: 0.1}
+      my_ur_shoulder_lift_joint: {trajectory: 0.2, goal: 0.1}
+      my_ur_elbow_joint: {trajectory: 0.2, goal: 0.1}
+      my_ur_wrist_1_joint: {trajectory: 0.2, goal: 0.1}
+      my_ur_wrist_2_joint: {trajectory: 0.2, goal: 0.1}
+      my_ur_wrist_3_joint: {trajectory: 0.2, goal: 0.1}
+
+forward_velocity_controller:
+  ros__parameters:
+    joints:
+      - my_ur_shoulder_pan_joint
+      - my_ur_shoulder_lift_joint
+      - my_ur_elbow_joint
+      - my_ur_wrist_1_joint
+      - my_ur_wrist_2_joint
+      - my_ur_wrist_3_joint
+    interface_name: velocity
+
+forward_position_controller:
+  ros__parameters:
+    joints:
+      - my_ur_shoulder_pan_joint
+      - my_ur_shoulder_lift_joint
+      - my_ur_elbow_joint
+      - my_ur_wrist_1_joint
+      - my_ur_wrist_2_joint
+      - my_ur_wrist_3_joint