Document VPC Routing Profile Changes

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Operators need a concise procedure for changing a VPC's routing profile and deciding when it is safe to release the previous VNI.

What This Involves

• Document the required VPC, routing profile, resource pool, and VNI checks.
• Document automatic and exact VNI selection through nico-admin-cli and the provider administrator REST endpoint.
• Document how to observe site and DPU convergence.
• Document how to reverse the change while the previous VNI remains reserved.
• Explain that releasing the previous VNI ends the guarantee that the VPC can return to that exact VNI.
• Explain that changing to EXTERNAL does not assign a public address or configure network security rules.
• Update the generated command and REST reference pages in this documentation-only change.

Depends on #5817 and #5818.

Part of #5813.