Hi Thibault!
Thanks for bringing this up. Swift-DocC does indeed support building standalone documentation catalogs without an associated Swift module. There is an example of this in the swift-docc repo. You bring up a very relevant point about documenting this use case; you can find documentation for building documentation for Swift frameworks in DocC’s documentation (see Build and Read Documentation for Swift-DocC) but it would be great to have docs for building standalone docs as well. Would you like to create a ticket on bugs.swift.org, or even open a PR? 
Thank you!