Sadly, this does not work for me, and there's absolutely ZERO indication, as to why.
Even the sample Apple SlothCreator package example does not work.
I am using the latest ship Xcode (13.2.1), and I downloaded the sample, fifteen minutes ago. I followed the example in this page, verbatim.
It simply opens the Apple documentation viewer, with the standard built-in docs. My app's docs are nowhere to be seen.
I have been writing frameworks and packages in Swift for years, and documenting with Jazzy, so all my code already has the triple-slash/double-star markdown notation.
I can't get documentation to appear for any of my projects (they have all been upgraded to 5.5, for some time).