Add KDocs for function `Cast` and `CastTo`

[zaleslaw]

https://github.com/Kotlin/dataframe/blob/master/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/cast.kt

Also, both functions wery bad tested, presented on site and examples are far from good situation
https://kotlin.github.io/dataframe/cast.html

Please align them with others

• Unit tests
• Example on site
• KDocs