feat: basic Toolkit styling for CMS markdown

[bodom0015]

Problem

Styling for Guides markdown is not consistent with other products

For example, the SDOH Toolkit has custom styling for things like headers, lists, tables, etc

Fixes #326

Approach

• feat: ported some styles over to Guides (hopefully without affecting other views like News or Showcases)
  • Basic styling for headers, lists, and accordions (details + summary)
  • Styling for table rows/headers/borders - something is interfering with font-weight, and I'm not sure what 🤔

How to Test

1. Navigate to https://deploy-preview-327--cheerful-treacle-913a24.netlify.app/guides/public-transit-equity
   • You should see new styling here that more closely matches what we see in the SDOH Toolkit
   • Compare with production: https://sdohplace.org/guides/public-transit-equity

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