refactor tutorial for underlays
Tutorial for Underlay and UnderlayDefinition
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-Insert a PDF, DWF, DWFx or DGN file as drawing underlay, the underlay file is NOT embedded into the DXF file::
+This example shows hot to insert a a PDF, DWF, DWFx or DGN file as drawing
+underlay. Each UNDERLAY entity requires an associated UNDERLAYDEF entity in the
+objects section, which stores the filename of the linked document and the
+parameters of the underlay. Multiple UNDERLAY entities can share the same
+UNDERLAYDEF entity.
+
+.. important::
+
+ The underlay file is NOT embedded into the DXF file:
+
+.. code-block:: Python
import ezdxf
