docs: add examples for index.ts files on server routes (#23390)

Co-authored-by: Sébastien Chopin <seb@nuxtlabs.com>
Co-authored-by: Sébastien Chopin <seb@nuxt.com>

docs/2.guide/2.directory-structure/1.server.md

@@ -180,6 +180,38 @@ Given the example above, fetching `/test` with:
 - **POST** method: Returns `Test post handler`
 - Any other method: Returns 405 error
 
+You can also use `index.[method].ts` inside a directory for structuring your code differently.
+
+**Example:**
+
+```ts [server/api/foo/index.ts]
+export default defineEventHandler((event) => => {
+  // handle the `api/foo` endpoint
+})
+```
+
+This is useful to create API namespaces.
+
+**Example:**
+
+```ts [server/api/foo/index.get.ts]
+export default defineEventHandler((event) => => {
+  // handle GET requests for the `api/foo` endpoint
+})
+```
+
+```ts [server/api/foo/index.post.ts]
+export default defineEventHandler((event) => => {
+  // handle POST requests for the `api/foo` endpoint
+})
+```
+
+```ts [server/api/foo/bar.get.ts]
+export default defineEventHandler((event) => => {
+  // handle GET requests for the `api/foo/bar` endpoint
+})
+```
+
 ### Catch-all Route
 
 Catch-all routes are helpful for fallback route handling. For example, creating a file named `~/server/api/foo/[...].ts` will register a catch-all route for all requests that do not match any route handler, such as `/api/foo/bar/baz`.