Hey Gwendal!
All good questions, but I think they’re mostly answered by this feature being opt-in by package authors. If we don't find a .spi.yml file with the relevant keys, we don't generate docs.
As Sven mentioned above we’re going to be adding a feature where authors can tell us where their official docs hosting is and we’ll send people there. We don’t yet have issues created for everything that’s going to make up the “completed” feature (is there ever such a thing?), but this is certainly one that we’ll implement.