Add Hugo build tests to docs content pipelines

Docs content pipelines need to run a Hugo build as a test to validate shortcodes.

We also need to test file naming. The move to Hugo, and dropping the ee prefix for GitLab project paths, introduces a few potential problems caused by filenames:

1. Index pages should be named _index.md rather than the standard index.md. Missing the underscore will cause sub-pages in this directory to not be built by Hugo (see https://gohugo.io/content-management/page-bundles/).

   • Possible solution: Add a check, probably in the lint-doc.sh script in the gitlab project, to make sure index files have the leading underscore.

2. The gitlab project must not include top-level directories with the same names as the other projects (runner, charts, etc). Otherwise the gitlab page will be built, but not the page for the other project.

   • Possible solution: include the --printPathWarnings --panicOnWarning flags on the Hugo build
   • Test for this in gitlab-org/gitlab!180801 (merged)

3. Update projects to use the markdownlint image from the docs-gitlab-com project.

Done when

• We have a Hugo build running as a test job on source content pipelines, configured to detect path warnings.
  • Operator
    • Add: gitlab-org/cloud-native/gitlab-operator!1065 (merged)
    • Enable: gitlab-org/cloud-native/gitlab-operator!1080 (merged)
  • Charts
    • Add: gitlab-org/charts/gitlab!4118 (merged)
    • Enable: gitlab-org/charts/gitlab!4124 (merged)
  • Omnibus
    • Add: gitlab-org/omnibus-gitlab!8162 (merged)
    • Enable: gitlab-org/omnibus-gitlab!8171 (merged)
  • Runner
    • Add + enable: gitlab-org/gitlab-runner!5306 (merged)
  • gitlab
    • Add + enable: gitlab-org/gitlab!180806 (merged)
• We've added a script to check for index.md files in source project pipelines. Until we do https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/issues/82, this check will fail, but that's OK (we can allow the job to fail for now).

We will introduce these as manual jobs so that they don't slow down regular docs pipelines, then enable them to run on regular docs pipelines.