Doc of `smoothness()` looks quite empty

[etiennebacher]

The current documentation of smoothness() looks quite empty:

I'm not familiar with it but is it possible to add a bit more details on the methods and some references other than a StackExchange post? For example, skewness() and kurtosis() are much more documented.

[IndrajeetPatil]

@DominiqueMakowski bump

[DominiqueMakowski]

Will do, thanks for the bump!

We could potentially relocate to data wizard along with other variable descriptors (albeit smoothness describes sequential series but still) Lol my bad I thought we were in modelbased ^^