Document x-group extension and MCP skill resources (#5299)

* Document x-group extension and MCP skill resources

Generated-By: mintlify-agent

* Apply suggestion from @ethanpalm

---------

Co-authored-by: mintlify[bot] <109931778+mintlify[bot]@users.noreply.github.com>
Co-authored-by: Ethan Palm <56270045+ethanpalm@users.noreply.github.com>

Author: mintlify[bot]

api-playground/openapi-setup.mdx

@@ -259,6 +259,34 @@ Set the URL of the autogenerated endpoint page using `x-mint: href`. When `x-min
 }
 ```
 
+### Group display names
+
+Set a custom display name for a tag's navigation group using the `x-group` extension on a tag object. By default, Mintlify uses the tag `name` as both the navigation group label and the URL path segment. The `x-group` extension overrides the group label while keeping the tag name for the URL.
+
+This is useful when you want a human-readable group name that differs from the tag used in your API paths.
+
+```json {4-9}
+{
+  "tags": [
+    {
+      "name": "user-management",
+      "description": "Endpoints for managing users",
+      "x-group": "User Management"
+    }
+  ],
+  "paths": {
+    "/users": {
+      "get": {
+        "tags": ["user-management"],
+        "summary": "List users"
+      }
+    }
+  }
+}
+```
+
+In this example, the navigation group displays as "User Management" but the generated page URL still uses the `user-management` tag name as its path segment.
+
 ## Auto-populate API pages
 
 Add an `openapi` field to any navigation element in your `docs.json` to automatically generate pages for OpenAPI endpoints. You can control where these pages appear in your navigation structure, as dedicated API sections or with other pages.