Rename exercises dir

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

topics/terraform/exercises/launch_ec2_instance/exercise.md

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+# Launch EC2 instance
+
+## Reuqirements
+
+* AWS account
+
+## Objectives
+
+1. Write Terraform configuration for launching an EC2 instance
+2. Run the commands to apply the configuration and create the EC2 instance
+3. What happens if you run again `terraform apply`?
+4. Destroy the instance you've created with Terraform

topics/terraform/exercises/launch_ec2_instance/solution.md

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+# Launch EC2 instance
+
+## Reuqirements
+
+* AWS account
+
+## Objectives
+
+1. Write Terraform configuration for launching an EC2 instance
+2. Run the commands to apply the configuration and create the EC2 instance
+3. What happens if you run again `terraform apply`?
+4. Destroy the instance you've created with Terraform
+
+## Solution
+
+```
+mkdir exercise
+
+cat << EOT >> main.tf
+terraform {
+  required_providers {
+    aws = {
+      source  = "hashicorp/aws"
+      version = "~> 4.16"
+    }
+  }
+
+  required_version = ">= 1.2.0"
+}
+
+provider "aws" {
+  region  = "us-west-2"
+}
+
+resource "aws_instance" "app_server" {
+  ami           = "ami-830c94e3"
+  instance_type = "t2.micro"
+
+  tags = {
+    Name = "ExampleAppServerInstance"
+  }
+}
+EOT
+
+terraform init
+terraform validate
+terraform plan
+
+# You should see this line at the end: Plan: 1 to add, 0 to change, 0 to destroy
+
+terraform apply -auto-approve
+
+# You should see the following output:
+# aws_instance.app_server: Creation complete after 49s [id=i-004651a9d4427d236
+
+# Running 'terraform apply' again won't change anything as
+# Terraform will compare actual infrastructure to your
+# configuration and won't find any difference. You should see the following line:
+# Apply complete! Resources: 0 added, 0 changed, 0 destroyed.
+
+# Remove instance
+terraform destroy -auto-approve
+
+# Destroy complete! Resources: 1 destroyed.
+```