Add changelog

Documentation/release/dev/htg-geometry-fill-material.md

+## Add FillMaterial option to HTG Geometry filter
+
+`vtkHyperTreeGridGeometry` now has a `FillMaterial` option.
+Enabled by default, the filter produces the same result as before.
+When disabled, only the interface lines are added to the resulting polydata (nothing where there are no interfaces or on the edges).