docs(pages): add design system for the walkthrough site

Writes down the two density tiers (Decision / Scan), the contrast
ladder, component patterns (step indicator, two-column row, chips,
footer), worked before/after examples, and anti-patterns. Derived
from two Socket.dev wizard screenshots so every page we ship to
GitHub Pages feels like Socket.

Targeted at junior contributors — explains the why behind every
rule, not just the what. Includes copy-paste CSS snippets using
the tokens already defined in walkthrough-overrides.css.

Author: jdalton