[PLUGIN REPORTS] - Tutorial for OpenApi plugin is a bit misleading

I've got a report that examples related to Kotlin + Maven setup in this tutorial https://javalin.io/tutorials/openapi-example are not really working:

* https://github.com/javalin/javalin-openapi/issues/234

I've prepared an example project to make sure it works fine on my end:

* https://github.com/javalin/javalin-openapi/tree/main/examples/javalin-maven-kotlin

After checking the tutorial, it only covers `maven-compiler-plugin`, but not `kotlin-compile-plugin` (even tho Kotlin sources are presented as an example). So we should add that missing snippet and maybe highlight at the top that for the most up-to-date examples it's better to check examples I maintain within the repo.