Add Korean (ko-kr) locale for contact-sales-gitlab page

this MR was AI generated

1. Change Summary

Adds Korean (ko-kr) localization for the /resources/contact-sales-gitlab/ page, including full locale infrastructure to support ko-kr as a new language across the site.

What changed:

• Added ko-kr locale to nuxt.config.ts and LANG_OPTIONS in common/constants.ts
• Created content/ko-kr/resources/contact-sales-gitlab.yml with Korean-translated page content (title, accordion sections, form header)
• Created content/shared/ko-kr/main-footer.yml (English base, required by the no-nav-minimal-footer layout)
• Created i18n/locales/ko-kr/index.yml (English base for UI strings)
• Created i18n/locales/ko-kr/marketo.yml with Korean translations for form fields used by form 28609 (field labels, dropdown options, error messages)
• Added missing formName: sales to en-us, ja-jp, and ko-kr contact-sales-gitlab.yml files (fixes Marketo form warning across all locales)
• Related https://gitlab.com/gitlab-com/marketing/marketing-operations/-/work_items/11617#note_3281667763

2. QA Checklist

• Code Cleanup: Any messages, linter warnings, and/or deprecation warnings are cleaned up in the console
• Efficient Code Review: I have tested and reviewed my own changes thoroughly before assigning a reviewer
• Localization checked for regressions

Steps to test:

1. Start dev server with pnpm dev
2. Navigate to http://localhost:3000/ko-kr/resources/contact-sales-gitlab/
3. Verify Korean page content renders (title, accordion, form header)
4. Verify Marketo form fields display Korean labels (First name → 이름, etc.)
5. Verify language selector appears in footer and includes 한국어
6. Verify English version at /resources/contact-sales-gitlab/ still works correctly
7. Verify no formName warning in console for any locale

Review App

Production	Review app
https://about.gitlab.com/resources/contact-sales-gitlab/	https://11617-form-request-contact-us-kr-localisation-for-paid-search.about.gitlab-review.app/ko-kr/resources/contact-sales-gitlab/