Replace Suisse fonts with Stacks design system fonts

[ekiefl]

Summary

Replaces the Suisse font family with new fonts that align with the Stacks design system:

• Merriweather (serif) — titles, h1 headings, summary body
• Atkinson Hyperlegible Next (sans-serif) — body text, h2 headings, captions, navbar
• Chivo Mono (monospace) — code, table of contents, frontmatter labels ("Summary", "Published", "Show Authors")

Fonts are loaded from Google Fonts instead of self-hosted woff2 files from arcadiascience.com.

What changed

• Removed all 8 @font-face declarations for Suisse fonts
• Added fonts.html to load Google Fonts via <link> tag
• Simplified CSS variables from semantic per-element names (--nb-header-font-family, --nb-banner-title-font-family, etc.) to three base font stacks (--nb-font-serif, --nb-font-sans, --nb-font-mono)
• Split h1/h2 styling — h1 is now serif, h2 is now sans-serif bold (previously both used the same serif font)
• Added four new color variables for font colors (--arcadia-body-color, --arcadia-label-color, --arcadia-muted-color, --arcadia-link-color) — these are from the Stacks design system and may change

How to preview locally

See the "Make and test your changes" section of the README.

After merging

This will be released as v2.0.0. Once released, theme update PRs will automatically open in all notebook pubs.