Add docs Hugo build test

What does this MR do?

1. Adds a docs-only CI job to test that docs pages can build on Hugo. This test validates file structure and shortcode syntax for the new website.

   • Example job: https://gitlab.com/gitlab-org/gitlab/-/jobs/9135464422

2. Updates references to the lint-markdown Docker image to reflect its new location.

   • docs code_quality: https://gitlab.com/gitlab-org/gitlab/-/jobs/9079105618
   • docs-lint links: https://gitlab.com/gitlab-org/gitlab/-/jobs/9079105623
   • docs-link markdown: https://gitlab.com/gitlab-org/gitlab/-/jobs/9079105616

This test is being added to every project that publishes to docs.gitlab.com and is required for the new Docs site to launch.

Related issues

• Add Hugo build tests to docs content pipelines (gitlab-org/technical-writing/docs-gitlab-com#161 - closed) • Sarah German
• Very similar MR for gitlab-runner: gitlab-runner!5306 (merged). This same test has been running on docs MRs in that project this week without issues.

MR acceptance checklist

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