Design: Replace generic docs typography with a distinctive font system

[hdcodedev]

Summary

Replace generic typography with a distinctive, intentional font system that aligns with the new frontend-design skill.

Why

Current docs use Inter/Roboto/system fallbacks, which conflicts with the skill guideline to avoid generic typography.

Scope

• Pick and document one display font + one body font pair for docs.
• Replace existing font imports and CSS tokens.
• Ensure headings/body/code hierarchy feels intentional and consistent.
• Validate readability at mobile and desktop breakpoints.

Acceptance Criteria

• Inter/Roboto/system are removed from default docs typography.
• globals.css typography tokens map to the new pair.
• app/layout.tsx font loading matches the new tokens.
• No layout regressions on docs pages and API page.

[hdcodedev]

Closed — addressed or superseded by modernization work in #44.