Add external javadoc links

build.gradle

@@ -47,6 +47,8 @@ allprojects {
         options {
             destinationDir(file("docs"))
             addBooleanOption('html5', true)
+            links "https://jd.adventure.kyori.net/api/$adventureVersion/"
+            links "https://docs.oracle.com/en/java/javase/11/docs/api/"
         }
     }