add: middleware.md

docs/manifest.json

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       "title": "Documentation",
       "heading": true,
       "routes": [
-        { "title": "はじめに", "path": "/docs/getting-started.md" },
+        {
+          "title": "はじめに",
+          "path": "/docs/getting-started.md"
+        },
         {
           "title": "Basic Features",
           "open": true,
@@ -89,6 +92,10 @@
             }
           ]
         },
+        {
+          "title": "Middleware",
+          "path": "/docs/middleware.md"
+        },
         {
           "title": "Deployment",
           "path": "/docs/deployment.md"
@@ -240,14 +247,20 @@
             }
           ]
         },
-        { "title": "よくある質問", "path": "/docs/faq.md" }
+        {
+          "title": "よくある質問",
+          "path": "/docs/faq.md"
+        }
       ]
     },
     {
       "title": "API Reference",
       "heading": true,
       "routes": [
-        { "title": "CLI", "path": "/docs/api-reference/cli.md" },
+        {
+          "title": "CLI",
+          "path": "/docs/api-reference/cli.md"
+        },
         {
           "title": "Create Next App",
           "path": "/docs/api-reference/create-next-app.md"
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       ]
     }
   ]
-}
+}

docs/middleware.md

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+---
+description: Learn how to use Middleware in Next.js to run code before a request is completed.
+---
+
+# Middleware
+
+<details open>
+  <summary><b>Version History</b></summary>
+
+| Version   | Changes                                                                                    |
+| --------- | ------------------------------------------------------------------------------------------ |
+| `v12.0.9` | Enforce absolute URLs in Edge Runtime ([PR](https://github.com/vercel/next.js/pull/33410)) |
+| `v12.0.0` | Middleware (Beta) added.                                                                   |
+
+</details>
+
+Middleware enables you to use code over configuration. This gives you full flexibility in Next.js, because you can run code before a request is completed. Based on the user's incoming request, you can modify the response by rewriting, redirecting, adding headers, or even streaming HTML.
+
+## Usage
+
+1. Install the latest version of Next.js:
+
+```jsx
+npm install next@latest
+```
+
+2. Then, create a `_middleware.ts` file under your `/pages` directory.
+
+3. Finally, export a middleware function from the `_middleware.ts` file.
+
+```jsx
+// pages/_middleware.ts
+
+import type { NextFetchEvent, NextRequest } from 'next/server'
+
+export function middleware(req: NextRequest, ev: NextFetchEvent) {
+  return new Response('Hello, world!')
+}
+```
+
+In this example, we use the standard Web API Response ([MDN](https://developer.mozilla.org/en-US/docs/Web/API/Response)).
+
+## API
+
+Middleware is created by using a `middleware` function that lives inside a `_middleware` file. Its API is based upon the native [`FetchEvent`](https://developer.mozilla.org/en-US/docs/Web/API/FetchEvent), [`Response`](https://developer.mozilla.org/en-US/docs/Web/API/Response), and [`Request`](https://developer.mozilla.org/en-US/docs/Web/API/Request) objects.
+
+These native Web API objects are extended to give you more control over how you manipulate and configure a response, based on the incoming requests.
+
+The function signature:
+
+```ts
+import type { NextFetchEvent } from 'next/server'
+import type { NextRequest } from 'next/server'
+
+export type Middleware = (
+  request: NextRequest,
+  event: NextFetchEvent
+) => Promise<Response | undefined> | Response | undefined
+```
+
+The function can be a default export and as such, does **not** have to be named `middleware`. Though this is a convention. Also note that you only need to make the function `async` if you are running asynchronous code.
+
+[Read the full Middleware API reference.](/docs/api-reference/next/server.md)
+
+## Examples
+
+Middleware can be used for anything that shares logic for a set of pages, including:
+
+- [Authentication](https://github.com/vercel/examples/tree/main/edge-functions)
+- [Bot protection](https://github.com/vercel/examples/tree/main/edge-functions)
+- [Redirects and rewrites](https://github.com/vercel/examples/tree/main/edge-functions)
+- [Handling unsupported browsers](https://github.com/vercel/examples/tree/main/edge-functions)
+- [Feature flags and A/B tests](https://github.com/vercel/examples/tree/main/edge-functions)
+- [Server-side analytics](https://github.com/vercel/examples/tree/main/edge-functions)
+- [Advanced i18n routing requirements](https://github.com/vercel/examples/tree/main/edge-functions)
+- [Logging](https://github.com/vercel/examples/tree/main/edge-functions)
+
+## Execution Order
+
+If your Middleware is created in `/pages/_middleware.ts`, it will run on all routes within the `/pages` directory. The below example assumes you have `about.tsx` and `teams.tsx` routes.
+
+```bash
+- package.json
+- /pages
+    _middleware.ts # Will run on all routes under /pages
+    index.tsx
+    about.tsx
+    teams.tsx
+```
+
+If you _do_ have sub-directories with nested routes, Middleware will run from the top down. For example, if you have `/pages/about/_middleware.ts` and `/pages/about/team/_middleware.ts`, `/about` will run first and then `/about/team`. The below example shows how this works with a nested routing structure.
+
+```bash
+- package.json
+- /pages
+    index.tsx
+    - /about
+      _middleware.ts # Will run first
+      about.tsx
+      - /teams
+        _middleware.ts # Will run second
+        teams.tsx
+```
+
+Middleware runs directly after `redirects` and `headers`, before the first filesystem lookup. This excludes `/_next` files.
+
+## Deployment
+
+Middleware uses a [strict runtime](/docs/api-reference/edge-runtime.md) that supports standard Web APIs like `fetch`. This works out of the box using `next start`, as well as on Edge platforms like Vercel, which use [Edge Functions](http://www.vercel.com/edge).
+
+## Related
+
+<div class="card">
+  <a href="/docs/api-reference/next/server.md">
+    <b>Middleware API Reference</b>
+    <small>Learn more about the supported APIs for Middleware.</small>
+  </a>
+</div>
+
+<div class="card">
+  <a href="/docs/api-reference/edge-runtime.md">
+    <b>Edge Runtime</b>
+    <small>Learn more about the supported Web APIs available.</small>
+  </a>
+</div>