Add a ridgeline plot

https://nbviewer.org/github/JetBrains/lets-plot-kotlin/blob/master/docs/examples/jupyter-notebooks/f-4.2.0/ridgeline_plot.ipynb

Start by reviewing the referenced ridgeline_plot.ipynb example to understand the intended chart and then inspect the repository's existing plot implementations and notebook examples. Done means Kandy supports an equivalent ridgeline plot and the corresponding example is available, with relevant tests or validation updated if the project provides them.

Written by the indexing model from the issue text.