Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/aws/exercises/hibernate_instance/exercise.md

@@ -0,0 +1,8 @@
+## AWS EC2 - Hibernate an Instance
+
+### Objectives
+
+1. Create an instance that supports hibernation
+2. Hibernate the instance
+3. Start the instance
+4. What way is there to prove that instance was hibernated from OS perspective?