fix(docs): sync custom serializer translations

[chaokunyang]

Summary

• rebuild the branch directly on the latest fory-site main after docs: rename manual serializers to custom serializers fory#3896
• sync the current zh-CN C#, Rust, and Swift custom serializer guides with their canonical English owners
• keep filenames and document IDs on custom-serializers.md / custom_serializers, with no legacy manual serializer aliases or paths
• update translated links to use the Docusaurus document ID and add the canonical Chinese term 自定义序列化器
• align sum type with the existing compiler terminology 联合类型
• cancel superseded Deploy, Lint, and Check broken links runs for the same event and Git ref

Testing

• scoped Markdown lint and Prettier checks
• scoped YAML lint for all changed workflows
• stale path, ID, route, and terminology scans
• English/Chinese heading and code-fence structure comparison
• Deploy workflow Unified Docs sync
• npm run download-avatars
• npm run build
• npm run typecheck
• verified /, /docs, /zh-CN/docs, /blog, and the three zh-CN custom_serializers routes