Add support for "linear key insights" in modular topic pages

[ikesau]

The key insights block is quite a constraining format that our users don't tend to engage with much, according to our analytics.

We would like for authors to have a way to write out insights in a linear way within the topic-page template. Joe and I think it makes the most sense to accomplish this with simple archie columns and content, not a new component type (nor a linear: true flag in the {.key-insights} archie)

Figma

Scope

• Write new CSS to ensure that standard columns and content look the same as the figma designs, when on a modular topic page
• Ensure that when a "linear key insights" section exists (even though that will be a somewhat non-formalized section of text data) that the sticky nav still works.

Extracurricular

As part of this work, it might be worth it to bite the bullet and decompose the centered-article.scss file:

gdoc_post_base.scss
gdoc_post_article.scss
gdoc_post_topic_page.scss
etc...

as currently the style overrides per content type are somewhat separated but don't have a formal hierarchy.