chore(deps): update nextra monorepo to v4

Author: renovate[bot]

Diff for: packages/website/package.json

@@ -21,8 +21,8 @@
     "clsx": "2.1.1",
     "next": "15.1.6",
     "next-themes": "0.4.4",
-    "nextra": "3.3.1",
-    "nextra-theme-docs": "3.3.1",
+    "nextra": "4.2.15",
+    "nextra-theme-docs": "4.2.15",
     "postcss": "8.5.1",
     "postcss-lightningcss": "1.0.1",
     "react": "19.0.0",