📝(docs) init architecture documentation

- Add docs about architecture
- Add ADR about the CRDT choice

docs/architecture.md

@@ -0,0 +1,19 @@
+## Architecture
+
+### Global system architecture
+
+```mermaid
+flowchart TD
+    User -- HTTP --> Front("Frontend (NextJS SPA)")
+    Front -- REST API --> Back("Backend (Django)")
+    Front -- WebSocket --> Yserver("Microservice Yjs (Express)") -- WebSocket -->  CollaborationServer("Collaboration server (Hocuspocus)") -- REST API <--> Back
+    Front -- OIDC --> Back -- OIDC ---> OIDC("Keycloak / ProConnect")
+    Back -- REST API --> Yserver
+    Back --> DB("Database (PostgreSQL)")
+    Back <--> Celery --> DB
+    Back ----> S3("Minio (S3)")
+```
+
+### Architecture decision records
+
+- [ADR-0001-20250106-use-yjs-for-docs-editing](./adr/ADR-0001-20250106-use-yjs-for-docs-editing.md)