Draft: Omnibus: null - Add Japanese translations and CI verification for /doc-locale structure

What does this MR do?

This MR adds a Japanese translated documentation file in a new /doc-locale/ja-jp directory structure, enabling docs.gitlab.com/ja-jp. We're beginning with a single translated file to establish the directory structure, with more translations to follow.

The MR also:

1. Adds a new CI job docs-verify-i18n-paths to verify each localized file has a corresponding English original
2. Creates i18n-specific CI jobs that mirror existing doc CI:
   • docs-lint-i18n content
   • docs-lint-i18n markdown
   • docs-lint-i18n links
   • docs-lint-i18n hugo

Updates CODEOWNERS to designate @gitlab-com/localization/maintainers as owners of the new /doc-locale directory

Related issues

1. https://gitlab.com/gitlab-com/localization/docs-site-localization/-/issues/102 -> Primary issue with all context related to this work.
2. https://gitlab.com/gitlab-com/localization/docs-site-localization/-/issues/118 -> Engineering issue for this MR.

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• When ready for review, MR is labeled workflowready for review per the Distribution MR workflow

If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.