docs: visual explainer of v1.x to v3.1.1 evolution (#737)

* docs: add visual explainer of v1.x → v3.1.1 technical evolution

Self-contained HTML infographic covering all major architectural changes,
new features, and breaking changes across the three major version streams.
Created as a cheat sheet for Alex ahead of the spec-kitty team call.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: restyle evolution + backlog pages to spec-kitty design system

Replace blueprint dark theme with spec-kitty's own palette (baby-blue
background, creamy-white cards, sunny-yellow borders, grassy-green
headings). Add page nav with logo linking both docs. Support dark mode
via prefers-color-scheme. Remove Google Fonts dependency in favor of
system font stack.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: improve color contrast to pass WCAG AA on both doc pages

Darken --grassy-green (#7BB661 → #1f5a1f), --text-dim (#546e7a →
#37505e), and --v3 (#5a9a3e → #3d7a28) in light mode. All text-on-
background pairs now meet WCAG AA (4.5:1+). Dark mode was already
compliant.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: AlexU-A