Add KDocs for `corr`

Docs for the [`corr` operation](https://kotlin.github.io/dataframe/corr.html).

Should be relatively simple, they involve just the selection of columns (of which we have a template), plus a `with` statement. So make sure to write your own examples.

Also, it may require a Grammar DSL, but that's optional, as it's quite a small operation.

Related to https://github.com/Kotlin/dataframe/issues/811