How to show Groovy and Kotlin DSL samples side-by-side

Gradle 5.0 docs now include Kotlin DSL snippets alongside Groovy snippets. This post shows two practical techniques for displaying both DSL variants side-by-side: using HTML `<details>` and `<summary>` elements in GitHub READMEs (Markdown or Asciidoc), and a vanilla CSS/JavaScript tab-based language selector for web documentation that persists user preferences via localStorage.