The DocC flow today works with refining out content for symbols from the compiler in a symbolgraph, represented by the library swift-docc-symbolkit, after which DocC itself transforms that into JSON data structures expected for rendering the content with the DocC renderer.
I don't know the SQLsite stored proc language, but I expect it would map in a fairly straightforward way into the concepts already in SymbolKit, which most of the time is represented in a JSON data structure. That would be the "big lift" part of something like this in my mind, or at least where I'd start. Do the work to extract/generate a symbolgraph from the CG-SQL, then hand that off to DocC and see how it does when it's interpreted/rendered through that lens.
There's a bit of exploring of this exact process recently for refining the JSON data dump from Swift Argument Parser arguments, although nothing terribly solid to base your work from, and there's similar work I think reflected in the GSOC project to map OpenAPI concepts into a symbolgraph. That might be worth looking at, as it's investigating the space as well.