fix(docs): remove stale Go codegen translation (#473)

## Summary

- remove the orphaned current zh-CN Go code-generation page after its
English source was deleted
- sync the current zh-CN native-serialization and troubleshooting docs
with the English removals
- preserve the English and zh-CN 1.3.0 versioned code-generation
documentation

## Root cause

Docusaurus only registers a translated document when the matching
English source document exists. The English current Go `codegen.md` page
was removed, but the current zh-CN translation and a link to it
remained. That left `/zh-CN/docs/next/guide/go/codegen.md` without a
generated route and caused the zh-CN broken-link check to fail.

## Impact

This restores the production documentation build and removes stale
current documentation for a deleted Go feature without changing
historical versioned docs.

## Validation

- `npm run build -- --locale zh-CN`
- `npm run download-avatars`
- `npm run build`
- `npm run typecheck`
- verified `/`, `/docs`, `/zh-CN/docs`, `/blog`, and the original
failing native-serialization route return HTTP 200