Rename exercises dir

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

topics/aws/exercises/subnets/exercise.md

@@ -0,0 +1,23 @@
+## AWS VPC - Subnets
+
+### Requirements
+
+Single newly created VPC
+
+### Objectives
+
+1. Create a subnet in your newly created VPC
+  1. CIDR: 10.0.0.0/24
+  2. Name: NewSubnet1
+2. Create additional subnet
+  1. CIDR: 10.0.1.0/24
+  2. Name: NewSubnet2
+  3. Different AZ compared to previous subnet
+3. Create additional subnet
+  1. CIDR: 10.0.2.0/24
+  2. Name: NewSubnet3
+  3. Different AZ compared to previous subnets
+
+## Solution
+
+Click [here to view to solution](solution.md)