docs: add English Localized Strings Editor reference

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Summary

• Add the English translation of the Localized Strings Editor reference.
• Add the page to the English MkDocs navigation.

Verification

• Pass A and Pass B retry-1 independent readonly reviews passed.
• RU/EN non-strict MkDocs builds passed.
• Strict builds remain blocked by pre-existing nav configuration warnings.
• REDkit runtime test was not run.

Sources are cited in the documentation page.