I18n docs encourage namespaced strings

What does this MR do and why?

Get the i18n docs to strongly encourage namespaced strings.

Most of the examples didn't actually include a namespace, so an agent following the vibe of the file is led to think that they're optional.

Add namespaces, and state clearly as a rule that namespaced helpers are to be used in all but the simplest cases.

See !254949 (comment 3975179236) where I forgot to namespace my strings again. This edit should flow into .ai/principles/distilled/internationalization.md and then .gitlab/duo/mr-review-instructions.yaml through distillation processes automatically within a week or two of merge.

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