Resolve "GitLab Docs Site: update article heading styles"

What does this MR do and why?

As first step of ✨ Redesign 2025 ✨, this MR updates list styling. See issue #362 (closed)

Screenshots, screen recordings, or links to review app

before	after

How to set up and validate locally

1. Configure a local GitLab Docs environment.
2. Check out this branch.

Examples in the review app:

• https://docs.gitlab.com/review-mr-839/administration/raketasks/check/
• https://docs.gitlab.com/review-mr-839/administration/settings/account_and_limit_settings/
• https://docs.gitlab.com/review-mr-839/ci/yaml/

Merge request acceptance checklist

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

• I have evaluated the MR acceptance checklist for this merge request.

gitlab-org/gitlab#513102 (closed)

Closes #362 (closed)