fix: Move 60 brand-guides redirects to docs/brand-guides/.gitbook.yaml

Redirects where both source and target belong to the Brand Guides
section are moved out of the root .gitbook.yaml into the section's
own config with prefixes stripped.

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

Author: louis-pre