documentation: propose kubeflow-system namespace migration

[aniketpati1121]

This PR adds a design proposal for gradually migrating Kubeflow control-plane
components from the hard-coded kubeflow namespace to kubeflow-system.

It outlines:

• Current namespace usage
• Risks and motivation for change
• Proposed component-by-component migration
• Testing and rollout strategy

[google-oss-prow]

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[juliusvonkohout]

I think there are more pressing issues such as #3319, #3317 and many more that i will prioritize.