Backport of "Ensure docs hugo_build CI job uses docs-gitlab-com stable branches"

What does this MR do and why?

Backports !200261 (merged) to 17.11. When testing the Docs site, we need to be sure to use the same version of the Hugo site as the docs we're testing against.

Otherwise we can block backport MRs by docs tests incorrectly failing.

• BEFORE: Failing job, incorrectly using the Hugo site's main branch: https://gitlab.com/gitlab-org/gitlab/-/jobs/11007618289
• AFTER: Passing job from this MR, correctly using the Hugo site's 17.11 branch: https://gitlab.com/gitlab-org/gitlab/-/jobs/11009463318

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.