Add a top-level description to the API reference

[globsterg]

https://kotlinlang.org/api/kotlinx-datetime/ looks like this:

https://kotlinlang.org/api/kotlinx.serialization/, https://kotlinlang.org/api/kotlinx.coroutines/, https://kotlinlang.org/api/latest/kotlin.test/, and https://kotlinlang.org/api/latest/jvm/stdlib/ all provide meaningful descriptions of the provided modules/packages and sometimes even a high-level overview, as well.