I'm not fully sure what you mean by "without referring to a particular instance of that community on a particular Lemmy instance". Are you trying to link to a community that will always open on the readers local instance? If so, then the way to do it is like this: [!pics@lemmy.ml](/c/pics@lemmy.ml). Results in: !pics@lemmy.ml.
Caveat: this link will only work on instances which have previously had at least one subscriber to the linked community.