Add docs Hugo build test

What does this MR do?

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

   • This job is configured to when: manual until we launch the Hugo site (Feb 16-17).
   • Example job: https://gitlab.com/gitlab-org/charts/gitlab/-/jobs/9078922001

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

   • check_docs_content: https://gitlab.com/gitlab-org/charts/gitlab/-/jobs/9078921998
   • check_docs_links: https://gitlab.com/gitlab-org/charts/gitlab/-/jobs/9078922000
   • check_docs_markdown: https://gitlab.com/gitlab-org/charts/gitlab/-/jobs/9078921999

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-operator: gitlab-org/cloud-native/gitlab-operator!1065 (merged). This test is being added to every repo that publishes to docs.gitlab.com.

Author checklist

For general guidance, please follow our Contributing guide.

Required

For anything in this list which will not be completed, please provide a reason in the MR discussion.

• 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.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

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.