docs: update endpoints to use file-routing approach (#5397)

- Move the original guides for creating endpoints and middlewares to sub-sections in the Endpoints category.
- Replace existing guides for endpoints and middlewares with the new approach.
- Update all endpoints-related snippets across docs to use this new approach.

www/apps/docs/content/development/fundamentals/local-development.md

@@ -104,7 +104,7 @@ yarn test
 
 ### Run API Integration Tests
 
-API integration tests are used to test out Medusa’s core endpoints.
+API integration tests are used to test out Medusa’s core API Routes.
 
 To run the API integration tests, run the following command in the root directory of the repository: