Add architecture documentation (#154)

backend/design/README.md

@@ -2,6 +2,7 @@
 
 ## Contents
 
+- [Architecture](./architecture.md): The architecture of CodePair's NestJS backend
 - [Yorkie Auth Webhook](./auth-webhook.md): Auth Webhook to prevent users from accessing unauthorized documents using a Public Key
 
 ## Maintaining the Document

backend/design/architecture.md

@@ -0,0 +1,33 @@
+# Yorkie Auth Webhook
+
+This document covers ㅅhe architecture of CodePair's NestJS backend.
+
+## Architecture
+
+Below is the diagram of the CodePair Backend Architecture. Currently, it adheres to the default architecture that can be generated through [NestJS CLI](https://docs.nestjs.com/cli/overview).  
+If you want to add a new component, you can use the command `nest generate controller [CONTROLLER_NAME]`.
+
+```
+                CodePair Backend
+ ┌───────┐      ┌──────────────────────────────────────────────────────────────────────┐
+ │Request├─────►│                                                                      │
+ └───────┘      │  ┌────┐     ┌─────┐     ┌──────────┐     ┌───────┐     ┌──────────┐  │
+                │  │Pipe│────►│Guard│────►│Controller│────►│Service│────►│Repository│  │
+┌────────┐      │  └────┘     └─────┘     └──────────┘     └───────┘     └──────────┘  │
+│Response│◄─────┤                                                                      │
+└────────┘      └──────────────────────────────────────────────────────────────────────┘
+
+```
+
+- Pipe: Handles data transformation/validation in the request/response pipeline.
+- Guard: Implements access control and validation logic for routes.
+- Controller: Defines the endpoints/routes and their corresponding request handling logic.
+- Service: Contains the business logic and performs operations required by the application.
+- Repository: Handles data persistence and retrieval operations from the data source.
+
+Note that the naming for the Repository Layer is `PrismaService`.
+
+## Note
+
+For now, we are using the architecture recommended by NestJS as a reference.  
+However, in the future, we can evolve towards implementing Clean Architecture by introducing interfaces between each layer.