docs(api): add addRouteMiddleware util (#6894)

Co-authored-by: Damian Głowala <[email protected]>

docs/content/3.api/3.utils/add-route-middleware.md

@@ -1,7 +1,76 @@
 # `addRouteMiddleware`
 
-::ReadMore{link="/guide/features/routing"}
+`addRouteMiddleware()` is a helper function to dynamically add route middleware in your Nuxt application.
+
+::alert{type=info}
+Route middleware are navigation guards stored in the [`middleware/`](/guide/directory-structure/middleware) directory of your Nuxt application (unless [set otherwise](/api/configuration/nuxt.config#middleware)).
 ::
 
-::NeedContribution
+## Usage
+
+```js
+addRouteMiddleware (name: string | RouteMiddleware, middleware?: RouteMiddleware, options: AddRouteMiddlewareOptions = {})
+```
+
+`addRouteMiddleware()` takes three arguments:
+
+- **name** `type: string | RouteMiddleware`
+
+`name` can be either a string or a function of type `RouteMiddleware`. Function takes the next route `to` as the first argument and the current route `from` as the second argument, both of which are Vue route objects.
+
+Learn more about available properties of [route objects](/api/composables/use-route).
+
+- **middleware** `type: RouteMiddleware`
+
+The second argument is a function of type `RouteMiddleware`. Same as above, it provides `to` and `from` route objects. It becomes optional if the first argument in `addRouteMiddleware()` is already passed as a function.
+
+- **options** `type: AddRouteMiddlewareOptions`  
+
+An optional `options` argument lets you set the value of `global` to `true` to indicate whether the router middleware is global or not (set to `false` by default).
+
+## Examples
+
+### Anonymous Route Middleware
+
+Anonymous route middleware does not have a name. It takes a function as the first argument, making the second `middleware` argument redundant:
+
+```ts [plugins/my-plugin.ts]
+export default defineNuxtPlugin(() => {
+  addRouteMiddleware((to, from) => {
+    if (to.path === '/forbidden') {
+      return false
+    }
+  })
+})
+```
+
+### Named Route Middleware
+
+Named route middleware takes a string as the first argument and a function as the second.
+
+When defined in a plugin, it overrides any existing middleware of the same name located in the `middleware/` directory:
+
+```ts [plugins/my-plugin.ts]
+export default defineNuxtPlugin(() => {
+  addRouteMiddleware('named-middleware', () => {
+    console.log('named middleware added in Nuxt plugin')
+  })
+})
+```
+
+### Global Route Middleware
+
+You can set an optional, third argument `{ global: true }` to indicate whether the route middleware is global:
+
+```ts [plugins/my-plugin.ts]
+export default defineNuxtPlugin(() => {
+  addRouteMiddleware('global-middleware', (to, from) => {
+      console.log('global middleware that runs on every route change')
+    },
+    { global: true }
+  )
+})
+```
+
+::ReadMore{link="/guide/features/routing"}
 ::