Use a new sample to illustrate the `fillNaNs` function in the website documentation

[Allex-Nik]

On the webpages https://kotlin.github.io/dataframe/columnselectors.html and https://kotlin.github.io/dataframe/fill.html#fillnans
the sample we use to illustrate the fillNaNs function does not contain any NaN values. Therefore, the iframes "Input", "Update", and "Output" in this sample are identical.
We need to change or replace the sample so that it contains NaN values, and the fillNaNs function actually affects it.