`dataFrameConfig.display.decimalFormat` does nothing for Swing table rendering

It's a bit misleading. We could update toJsonWithMetadata to use provided format to convert Double/Float values to String. But it should be disabled by default because converting Double/Float to String might remove some other functionality like sum on select, and so on. So it's more like a workaround. Proper implementation might have to be done on IJ side. However, if someone explicitly opts in, maybe it's ok?

Apart from that toJsonWithMetadata could also support cell renderers, for example

Related documentation pages:
https://kotlin.github.io/dataframe/jupyterrendering.html explicitly different from IJ rendering
https://kotlin.github.io/dataframe/rendering.html
https://kotlin.github.io/dataframe/kotlin-dataframe-features-in-kotlin-notebook.html

Start by reading updateToJsonWithMetadata and the linked Jupyter and rendering documentation. Clarify whether the scope is opt-in decimal formatting, cell-renderer support, or both, and whether the fix belongs in this project or on the IntelliJ side. Done should include an agreed default behavior and a documented way to enable the workaround.

Written by the indexing model from the issue text.