Document the MinIO Operator init container

[feorlen]

Operator uses an init container to perform some initialization for each new pod. There isn't any documentation about what it does, would be nice to have some.

Goals

• Basic info on what the Operator init container does.
• Maybe: additional detail on how to use your own (additional) init container.

Non-Goals

Detail on how to create the container itself.

Additional context

Kubernetes docs: https://kubernetes.io/docs/concepts/workloads/pods/init-containers/
@cniackz has some detail about the Operator container, not sure where.
Related to #967

[djwfyi]

Document as part of a k8s/Operator core concept doc.