docs: add Swagger UI

Summary:

Test Plan:

src/index.ts

@@ -1,3 +1,4 @@
+import { swaggerUI } from "@hono/swagger-ui";
 import { OpenAPIHono } from "@hono/zod-openapi";
 
 const app = new OpenAPIHono();
@@ -14,7 +15,7 @@ app.route(
 );
 
 // The OpenAPI documentation will be available at /doc
-app.doc("/doc", {
+app.doc("/openapi.json", {
   openapi: "3.0.0",
   info: {
     version: "1.0.0",
@@ -22,4 +23,6 @@ app.doc("/doc", {
   },
 });
 
+app.get("/docs", swaggerUI({ url: "/openapi.json" }));
+
 export default app;