docs: align zh docs with TRANSLATION_SCOPE policy

- Remove zh/exercises and zh/reference/api (English-only per policy)
- Update zh sidebar to link to English exercises/api
- Add clear navigation to English-only content in zh/index.md
- Fix dead links in VitePress config

Author: shijiashuai

docs/.vitepress/config.ts

@@ -206,11 +206,7 @@ export default defineConfig({
             {
               text: '练习',
               items: [
-                { text: '概述', link: '/zh/exercises/' },
-                { text: '内存优化', link: '/zh/exercises/module-02-memory' },
-                { text: 'SIMD', link: '/zh/exercises/module-04-simd' },
-                { text: '并发', link: '/zh/exercises/module-05-concurrency' },
-                { text: '解答', link: '/zh/exercises/solutions' },
+                { text: '练习（英文）', link: '/en/exercises/' },
               ],
             },
             {
@@ -219,9 +215,7 @@ export default defineConfig({
                 { text: '常见问题', link: '/zh/reference/faq' },
                 { text: '故障排查', link: '/zh/reference/troubleshooting' },
                 { text: 'API 入口', link: '/zh/reference/api-reference' },
-                { text: '内存工具', link: '/zh/reference/api/memory-utils' },
-                { text: 'SIMD 包装器', link: '/zh/reference/api/simd-wrapper' },
-                { text: '基准测试工具', link: '/zh/reference/api/benchmark-utils' },
+                { text: 'API 参考（英文）', link: '/en/reference/api/memory-utils' },
               ],
             },
             {