Added doc to inform about saving cost when using private subnets and AWS S3 cache

Kubecon 2022 challenge \o/ https://codechallenge.dev/j/kubeconeu

What does this MR do?

In our setup, we have noticed that some costs on AWS can be saved in this scenario by doing a proper configuration at the VPC (still reviewing the proper way to document this :-) )

More info at:

https://tomgregory.com/when-to-use-an-aws-s3-vpc-endpoint/

https://www.cloudzero.com/blog/reduce-nat-gateway-costs

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?