DXF export fails because of missing header.dxf and trailer.dxf

If I have understood correctly, GDAL is also compiled in DuckDB Spatial.

But there is a problem when we try to export to DXF, because DuckDB Spatial cannot find **header.dxf** and **trailer.dxf**.
It looks into GDAL-DATA environment variable to find the missing files.

I think it would be enough if this was mentioned in the documentation.

Here's a small example to use DuckDB with AutoCAD/BricsCAD: https://www.theswamp.org/index.php?topic=59448.0

No documentation file or test is named. Start by locating the DuckDB Spatial documentation for DXF export and verify that it explains the GDAL_DATA setting and the required header.dxf and trailer.dxf files; done means users can configure DXF export without the missing-file error.

Written by the indexing model from the issue text.