fix: Move brand-guides redirects, remove cross-section redirects

Move 60 brand-guides redirects to docs/brand-guides/.gitbook.yaml
with URL prefixes stripped. Remove 111 cross-section redirects that
were silently 404ing — imported as site-level redirects via GitBook UI.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: louis-pre