Moving middleware file (#36009)

This PR fixes a request from `@rauchg` that we specify more clearly that Middleware is Beta and organize it properly within the navigation. 

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`

docs/middleware.md → docs/advanced-features/middleware.md

@@ -2,7 +2,7 @@
 description: Learn how to use Middleware in Next.js to run code before a request is completed.
 ---
 
-# Middleware
+# Middleware (Beta)
 
 <details open>
   <summary><b>Version History</b></summary>

docs/manifest.json

@@ -143,8 +143,11 @@
           ]
         },
         {
-          "title": "Middleware",
-          "path": "/docs/middleware.md"
+          "path": "/docs/middleware",
+          "redirect": {
+            "destination": "/docs/advanced-features/middleware",
+            "permanent": true
+          }
         },
         {
           "title": "Going to Production",
@@ -263,6 +266,10 @@
               "title": "Measuring performance",
               "path": "/docs/advanced-features/measuring-performance.md"
             },
+            {
+              "title": "Middleware (Beta)",
+              "path": "/docs/advanced-features/middleware.md"
+            },
             {
               "title": "Debugging",
               "path": "/docs/advanced-features/debugging.md"