feat(build): add com and cn site build targets

- Rename the package and deployment domains to opensource.onixbyte.com
  and opensource.onixbyte.cn
- Extract a createConfig(target) factory in rspress.config.ts, with a
  thin rspress.cn.config.ts wrapper and build:com / build:cn scripts
- Make the cn build lead with zh-hans, add a Douyin social link, and
  omit international links such as X
- Emit canonical, hreflang alternate and Open Graph tags, plus a
  sitemap with per-route priorities and a generated robots.txt
- Remove the Gitea and GitLab CI pipelines and point the Vercel
  redirect at opensource.onixbyte.cn
- Drop the resolved Microsoft email delivery notice from the
  notifications pages

package.json

@@ -1,10 +1,12 @@
 {
-  "name": "blog.onixbyte.com",
+  "name": "opensource.onixbyte.com",
   "version": "1.0.0",
   "private": true,
   "type": "module",
   "scripts": {
     "build": "rspress build",
+    "build:com": "rspress build --config rspress.config.ts",
+    "build:cn": "rspress build --config rspress.cn.config.ts",
     "dev": "rspress dev",
     "format": "prettier --write .",
     "lint": "eslint .",
@@ -15,6 +17,8 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.39.4",
+    "@mdx-js/react": "3.1.1",
+    "@rspress/plugin-sitemap": "2.0.11",
     "@tailwindcss/postcss": "^4.3.0",
     "@types/node": "^22.8.1",
     "@types/react": "^19.2.14",
@@ -25,6 +29,7 @@
     "react": "^19.2.6",
     "react-dom": "^19.2.6",
     "tailwindcss": "^4.3.0",
+    "typescript": "6.0.3",
     "typescript-eslint": "^8.58.1"
   }
 }