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/copy-to-word.md

@@ -63,4 +63,6 @@ DOCX 导出与以下软件兼容：
 
 ---
 
+**延伸阅读：** [LaTeX vs MathML：应该选哪种格式？→](/blog/latex-vs-mathml)
+
 [尝试将公式导出到 Word →](/app)