docs: modernize platform capability and architecture guides to DBK Platform (#5)

Fixes #1

### Summary of Changes
- Modernized architecture documents to establish DBK Platform as the canonical multi-tenant commerce platform identity, treating HomAroma as the reference tenant.
- Updated frontend comparison to accurately reflect apps/spa (React 19 + Vite) vs apps/website (Next.js 15 SSR) instead of legacy names.
- Cleaned typos in PRISMA7_GUIDE and added cross-references to docs/PLATFORM_OVERVIEW.md.
- Updated docs/DOCUMENTATION_MIGRATION.md completion tracking.

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Co-authored-by: Your Name <you@example.com>
Reviewed-on: #5
Co-authored-by: antigravity-bot <bot@kittisak.com>

ADMIN_MCP_SECTION_GUIDE.md

@@ -1,6 +1,6 @@
 # DBK Platform Admin Section Guide
 
-> **Platform scope:** This guide describes shared CMS administration capabilities. Product and content examples are tenant-specific; HomAroma is the reference tenant.
+> **Platform scope:** This guide describes shared CMS administration capabilities. Product and content examples are tenant-specific; HomAroma is the reference tenant. See [`docs/PLATFORM_OVERVIEW.md`](docs/PLATFORM_OVERVIEW.md).
 
 This guide explains the dynamic page blocks available in the Page Builder and what each block contains.
 
@@ -138,7 +138,7 @@ This guide explains the dynamic page blocks available in the Page Builder and wh
 
 ### `SCENT_NOTES_BREAKDOWN`
 
-- Purpose: Fragrance layer breakdown
+- Purpose: Fragrance layer breakdown (reference tenant vertical block; pattern for multi-tier attribute breakdowns)
 - Contains:
   - `title` localized string optional
   - `description` localized string optional