Add layout explanatory table

Added an RST table for the dashboard layout.

Resources/doc/reference/dashboard.rst

@@ -288,13 +288,13 @@ Supported positions right now are the following:
 
 The layout is as follows:
 
-    TOPTOPTOPTOPTOPTOPTOPTOPTOPTOPTOPTOP
-    
-    LEFTLEFTLEF CENTERCENTE RIGHTRIGHTRI
-    LEFTLEFTLEF CENTERCENTE RIGHTRIGHTRI
-    LEFTLEFTLEF CENTERCENTE RIGHTRIGHTRI
-    
-    BOTTOMBOTTOMBOTTOMBOTTOMBOTTOMBOTTOM
++----------------------------------+
+|               Top                |
++--------+--------------+----------+
+| Left   |    Center    | Right    |
++--------+--------------+----------+
+|             Bottom               |
++----------------------------------+
 
 On ``top`` and ``bottom`` positions, you can also specify an optionnal ``class`` option to set the width of the block.