docs: Expand on project structure

docs/getting_started/PROJECT_STRUCTURE.md

@@ -44,3 +44,31 @@ infra/
   tests/                         Infra tests
   nix/                           Shared Nix configs
 ```
+
+When making a new endpoint, you'll need to add files in the following locations:
+
+1. **API Definition**: `fern/definition/<my-pkg>/<endpoint>/__package.yml`: This is where the API definition goes.
+
+1. **API Router**: `svc/api/<my-pkg>/src/route/mod.rs`: This might call an `op!` or a `msg!` from the business logic.
+
+1. **API Endpoint**: `svc/pkg/<my-pkg>/src/route/<endpoint>.rs`: The handler for the route goes here.
+
+1. **Business Logic**: `svc/pkg/<my-pkg>`: This is where the actual business logic goes.
+
+```mermaid
+sequenceDiagram
+    participant Client as Client
+    participant API_Router as API Router (svc/api/<my-pkg>/src/route/mod.rs)
+    participant API_Endpoint as API Endpoint (svc/pkg/<my-pkg>/src/route/<endpoint>.rs)
+    participant Business_Logic as Business Logic (svc/pkg/<my-pkg>)
+
+    Client->>API_Router: Request
+    API_Router->>API_Endpoint: Forward request
+    API_Endpoint->>Business_Logic: Call business logic
+    Business_Logic-->>API_Endpoint: Return result
+    API_Endpoint-->>API_Router: Return result
+    API_Router-->>Client: Response
+```
+
+Notes:
+- When working on endpoints, you need to include crates for adding ops, messages, and other dependencies.