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/image-to-latex.md

@@ -77,4 +77,6 @@ x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}
 
 ---
 
+**延伸阅读：** [3 秒从白板到 LaTeX：学生的高效工作流 →](/blog/student-workflow)
+
 准备好了吗？[立即上传公式图片 →](/app)