Added Oracle Java Docs for interfaces (#21602)

* Added Oracle Java Docs for interfaces

* Fixed formatting

guide/english/java/interfaces/index.md

@@ -262,3 +262,6 @@ public class SmartPhone implements MusicPlayer {
 Whoops, did I forget `next()` ? See, since it was a `default` method, I didn't had to provide an implementation at all. (Won't work for JDK < 8)
 
 So, now you have a good grasp of Interfaces! Go learn about Abstract Classes to know how Java gives you yet another way to define contracts.
+
+#### More Information
+- [Oracle Java Docs - Interfaces](https://docs.oracle.com/javase/specs/jls/se7/html/jls-9.html)