docs(routing): add middleware section

docs/content/1.guide/3.routing.md

@@ -149,3 +149,75 @@ export default defineNuxtConfig({
 })
 ```
 ::
+
+## Route Middleware
+
+Nitro route middleware can hook into the request lifecycle.
+
+::alert{type=primary}
+A middleware can modify the request before it is processed, not after.
+::
+
+Middleware are auto-registered within the `middleware/` directory.
+
+```md
+routes/
+  hello.ts
+middleware/
+  auth.ts
+  logger.ts
+  ...
+nitro.config.ts
+```
+
+### Simple Middleware
+
+Middleware are defined exactly like route handlers with the only exception that they should not return anything.
+
+```ts [middleware/auth.ts]
+export default defineEventHandler((event) => {
+  // Extends or modify the event
+  event.context.user = { name: 'Nitro' }
+})
+```
+
+::alert
+Returning anything from a middleware will close the request and should be avoided!
+::
+
+### Execution Order
+
+Middleware are executed in directory listing order.
+
+```md
+middleware/
+  auth.ts <-- First
+  logger.ts <-- Second
+  ... <-- Third
+```
+
+Prefix middleware with a number to control their execution order.
+
+```md
+middleware/
+  1.logger.ts <-- First
+  2.auth.ts <-- Second
+  3.... <-- Third
+```
+
+### Request Filtering
+
+Middleware are executed on every request.
+
+Apply custom logic to scope them to specific conditions.
+
+For example, you can use the URL to apply a middleware to a specific route:
+
+```ts [middleware/auth.ts]
+export default defineEventHandler((event) => {
+  // Will only execute for /auth route
+  if (getRequestURL(event).startsWith('/auth')) {
+    event.context.user = { name: 'Nitro' }
+  }
+})
+```