add(changelog): for improvements to `vtkCompositeArray`

Documentation/release/dev/improve-vtkCompositeArray.md

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+## Improve performance of `vtkCompositeArray`
+
+Flattening out the internal tree structure of the `vtkCompositeImplicitBackend` has increased access performance considerably when dealing with multi-level binary trees by avoiding intermediate calls through the `vtkImplicitArray`structure.
+
+This has led to a significant API change in the `vtkCompositeImplicitBackend` constructor which now takes a `std::vector<vtkDataArray*>` instead of two `vtkDataArray*`s directly leading to better quality of life for users as well.