[Cloud Marketplace] Update documentation with steps to deploy when AMI is selected as the delivery method and Launch through EC2 option is selected

I was trying to deploy couchbase on AWS market place using the following steps:

1. Go to https://aws.amazon.com/marketplace/pp/prodview-zy5g2wqmqdyzw and click "Continue to Subscribe"
2. Click Continue to Configuration in the next page
3. Select Amazon Machine Image as the Delivery method and click "Continue to Launch"
4. From the Choose Action menu select Launch through Ec2 and click launch

Now you are taken to the Ec2 launch dialog to launch the instance - continue through and launch - the instance comes up and I expected if I hit the 8091 port on the public IP of the instance, I will reach the admin console - but it did not work - on checkin with dev, they said that usually a CFT script is used and asked me to execute the following commands after sshing into the instance:

sudo bash /setup/postinstall.sh 0
sudo bash /setup/posttransaction.sh

After this, admin console was accessible. This is not documented anywhere. We should update https://docs.couchbase.com/server/current/cloud/couchbase-aws-marketplace.html to explain all the ways to launch couchbase instances and the steps tp be followed