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]

I have hereby read the F5 CLA and agree to its terms

[AliMahmoudDev]

Hi maintainers! Just a heads up that this branch may need a rebase with the latest main. Happy to address any feedback on the guide content. Thanks!

[mjang]

cc @kamalchaturvedi

[travisamartin]

Thanks for the contribution! One style note before merging: please remove the "Step x:" prefix from the subheadings (for example, "Step 1: Create a data plane key in the target namespace" → "Create a data plane key in the target namespace").

The F5 style guide recommends against numbering steps in headings. See: https://github.com/F5Docs/style-guide/blob/main/procedures/step-numbers-in-headings.md

[y82]

Looks good to me—thanks for the contribution!

[mjang]

Note: this has received internal approval (Slack) from the issue author, https://github.com/kamalchaturvedi

So once you've addressed @travisamartin 's request, we should be ready to merge. Thank you for your contribution!