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/en/2026-03-08-researcher-workflow.md

@@ -79,4 +79,6 @@ The real value of digitization compounds over time. A well-organized LaTeX refer
 
 Start with the past year's notebooks. The 7-hour investment pays dividends for years.
 
+**See also:** For PDF file limits, supported types, and export options, see the [PDF Extraction documentation →](/docs/pdf-extraction)
+
 [Start digitizing your notes →](/app)