Add documentation about aws s3 region endpoints to KOTS config dashboard

[mrsimonemms]

If using local registry with S3 backend AND using a region other than us-east-1, the URL must include the region.

For example, for eu-west-1, this would be s3.eu-west-1.amazonaws.com rather than s3.amazonaws.com

IMPORTANT this does not affect the main object storage and Minio seems to handle this automatically. Also, we shouldn't amend the preflight check as there are multiple ways for users to access their S3 backend

[mrsimonemms]

I have a feeling that this is already done

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.