docs: Add links to API reference in READMEs

Author: lambdalisue

echo-graphql/README.md

@@ -67,6 +67,8 @@ type Subscription {
 }
 ```
 
+See [docs/api.md](./docs/api.md) for detailed API reference.
+
 ## Features
 
 | Feature       | Description                                                                    |

echo-grpc/README.md

@@ -48,6 +48,8 @@ service Echo {
 }
 ```
 
+See [docs/api.md](./docs/api.md) for detailed API reference.
+
 ## Features
 
 | Feature                 | Description                                      |

echo-http/README.md

@@ -46,6 +46,8 @@ docker run -p 8080:8080 -v $(pwd)/.env:/app/.env ghcr.io/jsr-probitas/echo-http:
 | `/delay/{seconds}` | GET    | Echo after delay                          |
 | `/health`          | GET    | Health check                              |
 
+See [docs/api.md](./docs/api.md) for detailed API reference.
+
 ## Response Format
 
 ```json