docs: add doc on generating openapi spec for express backend (#1888)

* docs: add doc on generating openapi spec for express backend

* docs: create guides folder
- move express-openapi guide to guides folder

guides/express-openapi-spec.md

@@ -0,0 +1,163 @@
+# How to Generate OpenAPI Spec from an Express Backend
+
+The OpenAPI specification is a standard guideline for describing and documenting APIs. An OpenAPI specification document can be either a JSON or YAML file.
+
+## Generating OpenAPI
+
+You can automatically generate an OpenAPI spec in JSON format for your Express backend.
+
+Assume you have an Express backend with following set of routes:
+
+```js
+// src/routes/user.route.js
+
+const { Router } = require("express");
+const { createUser, deleteUser, getAllUsers, getUser, updateUser } = require("../controllers/user.controller");
+
+const userRoute = () => {
+  const router = Router();
+
+  router.post("/users", createUser);
+
+  router.get("/users", getAllUsers);
+
+  router.get("/users/:id", getUser);
+
+  router.patch("/users/:id", updateUser);
+
+  router.delete("/users/:id", deleteUser);
+
+  return router;
+};
+
+module.exports = { userRoute };
+```
+
+```js
+// src/routes/book.route.js
+
+const { Router } = require("express");
+const { createBook, deleteBook, getAllBooks, getBook, updateBook } = require("../controllers/book.controller");
+
+const bookRoute = () => {
+  const router = Router();
+
+  router.post("/books", createBook);
+
+  router.get("/books", getAllBooks);
+
+  router.get("/books/:id", getBook);
+
+  router.put("/books/:id", updateBook);
+
+  router.delete("/books/:id", deleteBook);
+
+  return router;
+};
+
+module.exports = { bookRoute };
+```
+
+Install the following dependencies:
+
+```bash
+npm install swagger-autogen
+npm install swagger-ui-express
+```
+
+In the root of your project, create a JSON file `swagger_output.json`. This file will contain the generated OpenAPI spec. Add some details about your server to the file in this format:
+
+```json
+ {
+  info: {
+    version: '',
+    title: '',
+    description: '',
+  },
+  host: '',
+  basePath: '',
+  schemes: [],
+  consumes: [],
+  produces: [],
+  tags: [
+    {
+      name: 'Users'
+    },
+    {
+    name: 'Books'
+     }
+  ],
+  securityDefinitions: {},
+  definitions: {},
+  components: {}
+};
+```
+
+The property `tags` represent a set of endpoints grouped together by their domain. The `Users` tag represent all the endpoints under the user routes.
+
+In the root of your project, create a file `swagger.js` and add the following content to it:
+
+```js
+const swaggerAutogen = require("swagger-autogen")();
+
+const outputFile = "./swagger_output.json";
+const endpointsFiles = ["./src/routes/user.route.js", "./src/routes/book.route.js"];
+
+swaggerAutogen(outputFile, endpointsFiles);
+```
+
+In the `swagger.js`, you import the swagger-autogen package that will handle generating the OpenAPI spec for your routes. The path to your routes and OpenAPI spec file are passed to the swagger-autogen function.
+
+Update your `package.json` with a script to generate the OpenAPI spec for your project.
+
+```json
+// package.json
+
+"scripts": {
+    "swagger-autogen": "node swagger.js"
+}
+```
+
+In your terminal, run `npm run swagger-autogen` to generate the OpenAPI spec. This command will update the `swagger_output.json` file that was previously created.
+
+Update your index file by importing the `swagger-ui-express` package, the `swagger_output.json` file. Mount the swagger ui middleware on the path `/documentation`
+. Your index file should look like this:
+
+```js
+// src/index.js
+
+const express = require("express");
+const dotenv = require("dotenv");
+const swaggerUi = require("swagger-ui-express");
+const swaggerFile = require("../swagger_output.json");
+
+const { connectToDatabase } = require("./databaseConnection");
+const { bookRoute } = require("./routes/book.route");
+const { userRoute } = require("./routes/user.route");
+
+dotenv.config();
+
+const HOST = process.env.HOST || "http://localhost";
+const PORT = parseInt(process.env.PORT || "4500");
+
+const app = express();
+
+app.use(express.urlencoded({ extended: true }));
+app.use(express.json());
+
+app.use("/", bookRoute());
+app.use("/", userRoute());
+app.use("/documentation", swaggerUi.serve, swaggerUi.setup(swaggerFile));
+
+app.get("/", (req, res) => {
+  return res.json({ message: "Hello World!🎉" });
+});
+
+app.listen(PORT, async () => {
+  await connectToDatabase();
+
+  console.log(`Application started on URL ${HOST}:${PORT} 🎉`);
+});
+```
+
+You can view the generated documentation for your routes when you navigate to the path `/documentation`.