Convert public directory from submodule to normal directory

- Remove public from git submodule tracking
- Add all public directory contents as regular files
- Fixes GitHub Actions error: "No url found for submodule path 'public' in .gitmodules"

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

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

public/book/1/ZGlkOnBsYzo0aHFqZm43bTZuNWhubzNkb2FtdWhnZWY/package.json

@@ -0,0 +1,8 @@
+{
+  "dependencies": {
+    "@gitbook-ng/gitbook": "^3.3.6",
+    "gitbook-plugin-codeblock-filename": "0.0.9",
+    "gitbook-plugin-diff": "^0.1.0",
+    "gitbook-plugin-mermaid-gb3": "^2.1.0"
+  }
+}