Relative spec links are broken on xregistry.io hosted versions

See first link in: https://xregistry.io/xreg/xregistryspecs/message-v1/docs/spec.html

If we want this to work then we may need to use absolute URLs instead. And then do we point to github's version or xregistry.io's version?

Or do we want to re-org the github directory structure so it work in both places? Probably flatten the dirs into one big one.