User Cluster Backup Location : Document how to use Minio S3 Endpoints

Currently a Minio S3 endpoint doesn't work out-of the box, see https://github.com/kubermatic/dashboard/issues/6732.

Right now it's possible to add additional parameters to the config for the Backup Storage Location (for AWS) Object via the ClusterBackupStorageLocation object (at the seed). This should be documented.

export KUBECONFIG=path-to-seed-kubeconfig
kubectl apply -f clusterbackuplocation.project.x.yaml

apiVersion: kubermatic.k8c.io/v1
kind: ClusterBackupStorageLocation
metadata:
  labels:
    csbl-display-name: seed-minio
    project-id: YOUR-PROJECT-ID
  name: seed-minio-YOUR-PROJECT-ID
  namespace: kubermatic
spec:
  backupSyncPeriod: 1m0s
  config:
    region: minio
    ####  NOTE: URL must be accisble from the user cluster network scope
    s3Url: https://minio.demo.kubermatic.io 
    # Whether to use path-style addressing instead of virtual hosted bucket addressing. Set to "true"
    # if using a local storage service like MinIO.
    #
    # Optional (defaults to "false").
    s3ForcePathStyle: "true"
    # If specified, use this instead of "s3Url" when generating download URLs (e.g., for logs). This
    # field is primarily for local storage services like MinIO.
    #
  credential:
    key: cloud-credentials
    name: credential-seed-minio-nczgr7n44g-h6qkj    #located in kubermatic namespace
  objectStorage:
    bucket: user-cluster-backup
  provider: aws