doc: clarify image display name uniqueness for all creation methods

[Hero2633]

Problem:

The Harvester documentation did not explicitly state that the Display Name of a VM image must be unique within the same namespace. This lack of clarity can lead users to use duplicate names, which may trigger finalizer deadlocks during image deletion, as identified in the core Harvester repository.

Solution:

Added a global :::important callout in the Upload Images guide. This notice clarifies the uniqueness requirement for the displayName field across all creation methods (URL, Local File, and Volume Export) to prevent potential operational conflicts.

Related Issue(s):

This documentation update is requested by maintainers in harvester/harvester#10113.

Test plan:

This is a documentation-only change. Verified the Markdown syntax and callout placement to ensure it renders correctly in the user guide.

Additional documentation or context

N/A

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