Simplify grid layout

What does this MR do and why?

Splitting layout-related changes from !61 (merged) to a separate MR for easier reviewing.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
2. Run make view
3. View an interior page at various breakpoints. Columns should be responsive and proportionately similar to the current site.

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.