Add docs-i18n-lint links CI job for anchor link validation

What does this MR do?

1. Expanded scripts/docs_i18n_verify_paths.sh to include the lychee check, renaming it docs_i18n_verify.sh, with two modes:
   • --verify-paths: Checks that all localized files have corresponding English originals
   • --check-links: Creates a temporary directory, generates symlinks, and runs lychee
2. Updates existing new docs-i18n-lint paths CI jobs:
   • To use docs_i18n_verify.sh --verify-paths
3. Adds a new docs-i18n-lint links CI job to validate anchor links in internationalized documentation by:
   • Creating a temporary test directory (/lychee_test_$timestamp) at the project root
   • Generating fallback symlinks for missing locale files to the original English docs
   • Running lychee with offline mode and fragment checking
   • Automatically cleaning up the temporary directory after testing
4. Enables markdownlint's MD051 (link-fragments) rule for /doc-locale to validate that internal anchor links point to existing headings.

To Test:

1. Check what locale directories exist ls -la doc-locale
2. Test path verification: ./scripts/docs_i18n_verify.sh --verify-paths
3. Test link checking: ./scripts/docs_i18n_verify.sh --check-links
4. See what symlinks get created by comment out the trap cleanup line: ls -la lychee_test_$timestamp
5. Add a bad internal link to any translated file in /doc-locale, and run ./scripts/docs_i18n.sh --check-links to see lychee fail

Test the markdownlint changes

cd doc-locale && markdownlint-cli2 --config .markdownlint/.markdownlint-cli2.yaml '**/*.md

Related issues

Related https://gitlab.com/gitlab-com/localization/docs-site-localization/-/issues/333

Author checklist

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Required

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated - TBA at https://docs.gitlab.com/development/documentation/testing/#language-specific-translation-linting once CI is added across projects
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened. gitlab-org/omnibus-gitlab!8802

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.