Document linking to URL fragment identifiers doesn't work in single page static build 

Arity-2 version of `n.clerk/doc-url` is only working in the default static build, but not with `:package` `:single-file` option. See https://snapshots.nextjournal.com/clerk/build/c92ebe569f706a99c45b569b5499d28ccdc7684f/index.html#/notebooks/markdown:appendix.

This is known since a while but we didn't have an issue for tracking this.