Add xref for definitions.

Author: gkellogg

index.html

@@ -44,6 +44,9 @@
       highlightVars:          true,
       pluralize:              true,
 
+      // Cross-reference definitions
+      xref: ["json-ld11", "json-ld11-api", "json-ld11-framing"],
+
       // editors, add as many as you like
       // only "name" is required
       editors:  [