docs: add pageExtensions note to Middleware and clarify default filename (#84045)

- Added a Note to the Middleware docs explaining that when
`pageExtensions` are customized (e.g. `.page.ts`), the Middleware file
should be named `middleware.page.ts` or `middleware.page.js`.
- Updated wording so `middleware.ts/js` is described as the default
filename, not a strict requirement.

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### What?

-Added a callout note to middleware.mdx stating that if pageExtensions
is customized (e.g., .page.ts), the Middleware file should also match
(middleware.page.ts/js).
-Updated the language from “must” → “default is” for filename clarity.


### Why?

-Developers customizing pageExtensions were misled by the existing docs,
which implied middleware.ts/js was always required.
-This PR prevents confusion when Middleware isn’t recognized after a
pageExtensions change.


### How?
Edited docs/app/api-reference/file-conventions/middleware.mdx.
Inserted a > **Note** block right after the paragraph explaining where
to create the middleware file.


Closes NEXT-
Fixes #84008

-->

---------

Co-authored-by: Joseph <[email protected]>

Author: Milancen123

docs/01-app/03-api-reference/03-file-conventions/middleware.mdx

@@ -20,6 +20,8 @@ Middleware executes before routes are rendered. It's particularly useful for imp
 
 Create a `middleware.ts` (or `.js`) file in the project root, or inside `src` if applicable, so that it is located at the same level as `pages` or `app`.
 
+If you’ve customized [`pageExtensions`](/docs/app/api-reference/config/next-config-js/pageExtensions), for example to `.page.ts` or `.page.js`, name your file `middleware.page.ts` or `middleware.page.js` accordingly.
+
 ```tsx filename="middleware.ts" switcher
 import { NextResponse, NextRequest } from 'next/server'