docs: added middleware documentation (#3000)

* docs: added a middleware documentation

* docs: fixes based on eslint

www/docs/sidebars.js

@@ -368,6 +368,11 @@ module.exports = {
           id: "advanced/backend/endpoints/add",
           label: "Create an Endpoint"
         },
+        {
+          type: "doc",
+          id: "advanced/backend/endpoints/add-middleware",
+          label: "Add a Middleware"
+        },
         {
           type: "doc",
           id: "advanced/backend/services/create-service",