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

rspress.cn.config.ts

@@ -0,0 +1,10 @@
+/**
+ * Build configuration for `onixbyte.cn`.
+ *
+ * Thin wrapper over the shared factory — see `rspress.config.ts` for what the
+ * `cn` target changes. Invoke it with `pnpm build:cn`, which passes this file
+ * via `rspress build --config`.
+ */
+import { createConfig } from "./rspress.config"
+
+export default createConfig("cn")