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>

goal.section.md

@@ -1,6 +1,6 @@
 # ระบบ CMS Pages และ Section Block ของ DBK Platform
 
-> **Platform scope:** HomAroma content blocks are reference-tenant examples, not a restriction on tenant verticals.
+> **Platform scope:** HomAroma content blocks are reference-tenant examples, not a restriction on tenant verticals. See [`docs/PLATFORM_OVERVIEW.md`](docs/PLATFORM_OVERVIEW.md).
 
 ## การทำงานของระบบ CMS Pages และ Section Block