docs: add guide for switching instances between namespaces

[AliMahmoudDev]

Summary

Adds a step-by-step guide for switching an NGINX instance from one namespace to another in NGINX One Console. The guide covers:

• Creating a data plane key in the target namespace
• Revoking the old data plane key
• Updating the NGINX Agent configuration with the new key
• Restarting the agent
• Verifying the instance appears in the new namespace

Also updates the glossary entry for "Namespace" to link to the new guide instead of just describing the process.

Closes #1339

[AliMahmoudDev]

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