ci: Validate redirects belong to their config's site section (#1094)

* ci: Validate redirects belong to their config's site section

The root .gitbook.yaml is rooted at docs/guides/, so redirects with
sources or targets in other sections (api/, brand-guides/) can't work
as space-level redirects. The validator now checks each redirect
belongs to its config's section and suggests using site-level
redirects in the GitBook UI for cross-section cases.

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

* 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>

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

Author: louis-pre