Hi! Unfortunately, it's out of the main scope for the foreseeable future, as we're focusing on K2 analysis and the new Gradle runner - both are vital to running Dokka, and we expect to be working on it until at least Dokka/Kotlin 2.0.
However, we will have someone from outside the team help us with improving our HTML format soon, so it might get addressed, but I can't promise anything :( Will keep it in mind when prioritizing issues
I don't remember right off the bat if there's plugin API for changing the navigation menu, but if you want - you can try to implement it as a 3rd party Dokka plugin, I'll give you pointers where to start.
As a side note: it looks as if it's easy to implement, but I think there will be some corner cases: the left sidebar is not resizeable, and some projects have a very deeply nested structure, so it can result in worse UX. To make it fully customizable through settings (i.e allow arbitrary grouping), the question is what configuration DSL to expose so that it's not error prone - all options I can think of come with tradeoffs. Also not sure about navigation itself - the actual page path needs to stay the same (be flat) for compatibility, and for external link resolution purposes it needs to be static - not sure how well it'll play with reordering things in the sidebar. So this issue is more effort than it seems, I'd say it's quite a big one :)