I am posting this as an answer, even though it is just another gripe. The reason is because I can't seem to track my comments; only answers. This is what I posted in a comment, above:
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 (From the page linked with instructions), 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).
I'm quite sure that I'm doing something wrong, but it is impossible to tell what it is, that I am doing wrong.