Disable Japanese for archives with separate config

What does this MR do and why?

Disables Japanese translations for archived documentation versions by using Hugo's environment-based configuration. Only the latest (unreleased) version on docs.gitlab.com will include Japanese translations.

• Created config/archive/hugo.yaml that defines only the en-us language
• Updated build-archive and view-archive Makefile targets to use --environment archive flag
• Hugo automatically loads configs from config/archive/ when using --environment archive
• No changes to the release process required

Closes gitlab-com/localization/docs-site-localization#744 (closed)

Why was this MR needed?

• Reduces scope and complexity for archived versions
• Eliminates the need to backport translation updates to release branches
• Simplifies the release process by not requiring translation management for stable versions
• Aligns with current production setup where translations have been disabled without issues

Screenshots, screen recordings, or links to review app

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How to set up and validate locally

1. Run make view-archive locally
2. Verify that only English content is built (check Hugo build output - should only show "EN - US" column)
3. Verify the archive site works correctly at http://localhost:1313/18.5/

Merge request acceptance checklist

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• I have evaluated the MR acceptance checklist for this merge request.