docs: add document for skip option of trailing slash middleware

docs/middleware/builtin/trailing-slash.md

@@ -55,6 +55,20 @@ app.get('/my-path/*', (c) => c.text('Wildcard route'))
 
 This option is available for both `trimTrailingSlash` and `appendTrailingSlash`.
 
+### <Badge type="info" text="optional" /> skip: `(path: string) => boolean`
+
+A function that determines whether the redirect should be skipped based on the request path. If the function returns `true`, the redirect will be skipped. This is useful when you want to exclude certain paths, such as those with file extensions, from being redirected.
+
+```ts
+app.use(
+  appendTrailingSlash({
+    skip: (path) => /\.\w+$/.test(path),
+  })
+)
+```
+
+This option is available for both `trimTrailingSlash` and `appendTrailingSlash`.
+
 ## Note
 
 It will be enabled when the request method is `GET` and the response status is `404`.