Document ECR repo location

As per discussion [here](https://github.com/bottlerocket-os/bottlerocket/pull/856#discussion_r393242181), we should think about documenting the ECR repo to allow customers to create their own set of scoped-down policies.

Start with the linked pull request discussion and inspect the website's existing documentation structure to find the appropriate page for this information. Done means the ECR repository location is documented clearly enough for customers to create scoped-down policies.

Written by the indexing model from the issue text.