Add sitemap

package.json

@@ -15,6 +15,7 @@
     "html-to-image": "^1.10.6",
     "next": "^12.1.6",
     "next-i18next": "^10.1.0",
+    "next-sitemap": "^3.1.22",
     "nprogress": "^0.2.0",
     "open-color": "^1.9.1",
     "ramda": "^0.27.1",
@@ -47,6 +48,7 @@
     "dev:web": "next dev -p 4000",
     "dev:assets": "http-server assets -p 4001 -s --cors",
     "build": "npm run compile:novels && next build",
+    "postbuild": "next-sitemap",
     "start": "next start",
     "lint": "next lint",
     "tsc": "tsc --watch",