Update documentation about ContainerRepository delete_failed status

In #480652 (closed), we set the status of container repositories that continuously fail deletions to delete_failed.

With this status, the container repository is no longer picked up by the background worker that checks for and delete container repositories tagged for deletion.

To include the container repository again to the pool of container repositories for deletion, the status of the ContainerRepository should be set to delete_scheduled.

For now, this should be done via the Rails console until there is enough need and request for #480653.

In this issue, we want to update the documentation and add instructions how to reset the container repository's status such that it will be picked up again for deletion