Javadoc plugin does not generate `overview-tree` page at all

**Technical details**
See https://github.com/Kotlin/dokka/blob/02f30b142aa467d3a24cc52a1fe3f2fed7ea1e33/plugins/javadoc/src/main/kotlin/org/jetbrains/dokka/javadoc/pages/htmlPreprocessors.kt#L42 (a root page is `RendererSpecificRootPage`)