Docs: add link to the docs site from /help landing page

What does this MR do?

Links the docs site from /help landing page. Hidden on docs.gitlab.com.

/help	docs.gitlab.com

cc/ @mikelewis @eread FYI

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8639

Review checklist

• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8639), CE-MR afterwards