docs: professionalize project site and add architecture diagram

[dev-ugurkontel]

Summary

Improve the Pages site so it feels like a professional product surface instead of a placeholder landing page, and add a real architecture diagram where contributors expect it.

What changed

• redesign the Pages site with a darker, more product-oriented visual language
• tighten the landing page copy around preview/staging security gating
• add a clearer delivery-flow section and stronger trust/integration framing
• add a Mermaid layered-architecture diagram to docs/ARCHITECTURE.md

Validation

• HTML parse check for docs/site/index.html
• relative Markdown link scan (ISSUES=0)