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

README.md

@@ -8,10 +8,10 @@ A static documentation site built with [Rspress](https://rspress.dev/), supporti
 
 ## URLs
 
-| Domain                                           | Recommended For                |
-|--------------------------------------------------|--------------------------------|
-| [onixbyte.cn](https://onixbyte.cn)               | Users in China mainland        |
-| [onixbyte.com](https://onixbyte.com)             | Users outside China mainland   |
+| Domain                                                     | Recommended For              |
+|------------------------------------------------------------|------------------------------|
+| [opensource.onixbyte.cn](https://opensource.onixbyte.cn)   | Users in China mainland      |
+| [opensource.onixbyte.com](https://opensource.onixbyte.com) | Users outside China mainland |
 
 ## Local Development