Rename exercises dir

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

topics/aws/exercises/ecs_task/exercise.md

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+## AWS Containers - Run Tasks
+
+Note: this costs money
+
+### Objectives
+
+Create a task in ECS to launch in Fargate.
+
+The task itself can be a sample app.

topics/aws/exercises/ecs_task/solution.md

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+## AWS Containers - Run Tasks
+
+Note: this costs money
+
+### Objectives
+
+Create a task in ECS to launch in Fargate.
+
+The task itself can be a sample app.
+
+### Solution
+
+#### Console
+
+1. Go to Elastic Container Service page
+2. Click on "Get Started"
+3. Choose "sample-app"
+4. Verify it's using Farget and not ECS (EC2 Instance) and click on "Next"
+5. Select "None" in Load balancer type and click on "Next"
+6. Insert cluster name (e.g. my_cluster) and click on "Next"
+7. Review everything and click on "Create"
+8. Wait for everything to complete
+
+1. Go to clusters page and check the status of the task (it will take a couple of seconds/minutes before changing to "Running")
+
+1. Click on the task and you'll see the launch type is Fargate