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/blog/zh/2026-03-08-researcher-workflow.md

@@ -79,4 +79,6 @@ TexPixel 改变了这个局面。
 
 从过去一年的笔记本开始。7 小时的投入，将带来多年的回报。
 
+**参考文档：** 关于 PDF 文件限制、支持类型和导出选项，请查看 [PDF 公式提取文档 →](/docs/pdf-extraction)
+
 [开始数字化你的笔记 →](/app)