[llvm] [Docs] Match body/toctree ordering on Reference and UserGuides (PR #195542)

[Alexis Engelke via llvm-commits]

aengelke wrote:

Any way of getting a nested navigation? I think this just solves the order, but not the missing hierarchy. (Maybe be outdenting everything else?) (I guess there's no chance to deduplicate with the existing lists without losing information?)

https://github.com/llvm/llvm-project/pull/195542