chore: Remove redundant mermaid files

- Remove mermaid.min.js and mermaid-init.js from root
  (mdbook-mermaid preprocessor generates these automatically)
- Clear additional-js in book.toml
- Add *.pdf to gitignore

Saves ~2.6MB from repository.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: GiggleLiu

.gitignore

@@ -60,3 +60,6 @@ book/
 
 # Tarpaulin
 tarpaulin-report.html
+
+# Generated documents
+*.pdf

book.toml

@@ -13,7 +13,7 @@ default-theme = "rust"
 git-repository-url = "https://github.com/liujinguo/problemreductions"
 edit-url-template = "https://github.com/liujinguo/problemreductions/edit/main/{path}"
 additional-css = []
-additional-js = ["mermaid.min.js", "mermaid-init.js"]
+additional-js = []
 no-section-label = false
 
 [output.html.fold]