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/en/pdf-extraction.md

@@ -72,4 +72,6 @@ Large PDFs with many pages can take 30–60 seconds. This is normal. The result
 
 ---
 
+**Further reading:** [I tried to extract formulas from my professor's PDF — real-world troubleshooting →](/blog/pdf-formula-issues)
+
 [Upload a PDF and extract formulas →](/app)