fix(site): restore brand and design system URLs

[bkearns]

Executive Summary

• Restore the brand guide and design system to their original public URLs: /brand.html and /design-system.html.
• Keep both pages unlinked from the homepage, navigation, and other site pages.
• Update legacy Ferrosa Memory forwarding pages and the design validation config to target the restored locations.

Verification

• Generated example docs: no content drift with Asciidoctor 2.0.20
• Docs navigation: 0 changed pages
• scripts/check-site.py: passed
• Design contrast: 183 combinations checked, 0 below AA
• git diff --check: passed
• Local HTTP checks: both restored pages return 200

CI Note

The macOS Chrome probe did not emit its expected sentinel across any configured page, including unchanged pages. The authoritative browser-layout gates run in CI against Ubuntu /usr/bin/google-chrome; all non-browser local gates passed.