refact: eliminate blog/docs content overlap

- Delete blog/copy-math-to-word (EN+ZH) — identical to docs/copy-to-word
- Rewrite blog/pdf-formula-issues as narrative troubleshooting story;
  operational steps now link out to docs/pdf-extraction
- Add "Further reading" cross-links: 4 docs → relevant blog posts
- Add "See also" cross-links: 3 blog posts → relevant docs

Docs = product reference; Blog = narrative/use cases/opinions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

content/docs/zh/pdf-extraction.md

@@ -72,4 +72,6 @@ PDF 可能已加密，公式可能以复杂矢量路径存储，或使用了非
 
 ---
 
+**延伸阅读：** [我试着从教授的 PDF 里提取公式——真实排障经历 →](/blog/pdf-formula-issues)
+
 [上传 PDF 提取公式 →](/app)