Add project documentation under Documentation/.

Covers architecture, setup, database schema, API reference, and image pipeline; link from README.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -2,6 +2,16 @@
 
 Interactive virtual art gallery: zoomable historical timeline, art movement bands, 3D navigable gallery halls, and painting influence graphs.
 
+## Documentation
+
+| Document | Purpose |
+|----------|---------|
+| [Documentation/basics.md](Documentation/basics.md) | Architecture, layout, user flow |
+| [Documentation/setup.md](Documentation/setup.md) | Install, env, npm scripts |
+| [Documentation/DB_structure.md](Documentation/DB_structure.md) | PostgreSQL tables |
+| [Documentation/API.md](Documentation/API.md) | REST endpoints |
+| [Documentation/data-and-images.md](Documentation/data-and-images.md) | Seeding and image pipeline |
+
 ## Stack
 
 - **Backend:** Node.js, Express, PostgreSQL