fix: deploy correct mdbook output directory (book/html)

The workflow was deploying ./book which doesn't contain index.html.
mdBook generates HTML files in ./book/html/ by default.

This fixes the 404 error on GitHub Pages.

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

Co-Authored-By: Claude <[email protected]>

Author: EvolveArt

.github/workflows/deploy-docs.yml

@@ -40,12 +40,12 @@ jobs:
         run: mdbook build
 
       - name: Add .nojekyll
-        run: touch book/.nojekyll
-      
+        run: touch book/html/.nojekyll
+
       - name: Deploy to GitHub Pages
         uses: peaceiris/actions-gh-pages@v3
         if: github.ref == 'refs/heads/main'
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
-          publish_dir: ./book
+          publish_dir: ./book/html