Modify troubleshooting & instructions for EventSystem (#249)

* Modify troubleshooting & instructions for EventSystem

* PR feedback: hierarchy clarification

* Precommit fixes

tutorials/pick_and_place/2_ros_tcp.md

 
    > Note: While using the ROS Settings menu is the suggested workflow as of this version, you may still manually create a GameObject with an attached ROSConnection component.
 
-1. Next, we will add a UI element that will allow user input to trigger the `Publish()` function. In the Hierarchy window, right click to add a new UI > Button. Note that this will create a new Canvas parent as well.
+1. Next, we will add a UI element that will allow user input to trigger the `Publish()` function. In the Hierarchy window, right click to add a new UI > Button. Note that this will also create a new Canvas parent, as well as an Event System.
 	> Note: In the `Game` view, you will see the button appear in the bottom left corner as an overlay. In `Scene` view the button will be rendered on a canvas object that may not be visible.
 
    > Note: In case the Button does not start in the bottom left, it can be moved by setting the `Pos X` and `Pos Y` values in its Rect Transform component. For example, setting its Position to `(-200, -200, 0)` would set its position to the bottom right area of the screen.
 - If Unity fails to find a network connection, ensure that the ROS IP address is entered correctly as the ROS IP Address in the RosConnect in Unity, and that the `src/niryo_moveit/config/params.yaml` values are set correctly.
 
 - If the ROS TCP handshake fails (e.g. `ROS-Unity server listening...` printed on the Unity side but no `ROS-Unity Handshake received` on the ROS side), the ROS IP may not have been set correctly in the params.yaml file. Try running `echo "ROS_IP: $(hostname -I)" > src/niryo_moveit/config/params.yaml` in a terminal from your ROS workspace.
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+- If the UI buttons appear to be unresponsive, such as not responding to clicks, ensure there is an [EventSystem](https://docs.unity3d.com/2020.1/Documentation/Manual/UIE-Events.html) in the scene hierarchy. This should be added automatically when adding UI elements, but if it is not, you can add one to your scene from the Hierarchy window via `(+) > UI > Event System`. You can also access this dropdown from right-clicking in an empty area in the Hierarchy window.
 
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tutorials/urdf_importer/urdf_appendix.md

     - limit
         - lower
         - upper
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 ## Disable Collision Support