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

vercel.json

@@ -10,7 +10,7 @@
           "value": "CN"
         }
       ],
-      "destination": "https://onixbyte.cn/:path*",
+      "destination": "https://opensource.onixbyte.cn/:path*",
       "permanent": false
     }
   ]