Provide multiplatform native example

[gabrielittner]

I have a project that has a shared nativeMain sourceSet that is build for multiple targets like iOS, tvOS, macOS, windows and Linux. All the documentation mentions are simple js and jvm examples where you have one sourceSet per target by default.

Describe the solution you'd like
A multiplatform example that includes multiple that includes native targets.

[IgnatBeresnev]

An example project has been added for multiplatform projects: https://github.com/Kotlin/dokka/tree/master/examples/gradle/dokka-multiplatform-example

And some points have been addressed in #2728