Simplify KDoc preprocess process

We should migrate to the easier `kodex {}` extension syntax.

As mentioned [here](https://github.com/Kotlin/dataframe/pull/959#discussion_r1854114787): we can change the publication task to include any sources we want, so in theory, we could just point at the generated sources of `processKDocsMain`.

We'd need to include both the compiled version of the generated sources (so `@ExcludeFromSources` keeps working) as well as a sources.jar built from it.