fix: docs/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-DIFF-14917201

Author: snyk-bot

docs/package.json

@@ -16,7 +16,7 @@
     "@heroicons/react": "~1.0.6",
     "next": "~12.2.5",
     "next-themes": "0.2.0",
-    "nextra": "2.0.0-alpha.56",
+    "nextra": "3.0.0",
     "nextra-theme-docs": "2.2.18",
     "react": "~18.2.0",
     "react-dom": "~18.2.0",