I've been working on an inline API documentation library for use with Ktor (a Kotlin/JVM server framework) recently - just published its first version!
It was a lot of fun to write - it benefits from a lot of Kotlin's language features - and using kotlinx.html to create the UI was a fairly good experience.
FYI, the response codes listed at the bottom of the page are links to https://http.cat/ - the best HTTP status code resource.