Add editor and viewer guidelines

Author: IsabelParedes

docs/use_editor.rst

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+URDF Editor
+===========
+
+To open the text editor for any URDF file, simply right click on the file and
+from the menu select "Open With" :math:`\rightarrow` "Editor".
+
+.. image:: _static/openEditor.png
+    :alt: Right click opens editor option

docs/use_viewer.rst

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+URDF Viewer
+===========
+
+The simplest way to open the viewer is by double clicking on any already 
+existing URDF file from the File Browser. New files can also be created from the
+top menu or from the launcher.
+
+With the URDF editor open, files can be modified directly and the changes will 
+be immediately reflected in the viewer.
+
+.. image:: _static/urdfEditor.gif
+    :alt: URDF live editor gif
+
+The viewer provides a control panel to manually modify the joint positions. 
+
+.. note::
+    The changes are only reflected in the viewer and are not saved to the URDF
+    file.
+
+.. image:: _static/urdfControls.gif
+    :alt: Gif moving the robot joints from the panel
+
+Only revolute and prismatic joints will be included in the panel, fixed joints 
+are automatically ignored. Each joint must have upper and lower joint limits to
+be displayed on the control panel.