Improve qsgen3 theme system: complete theme override and new minimal theme

- Remove legacy themes (black-orange, minimal) with old .tpl structure
- Create new minimal theme with proper qsgen3 structure:
  - Complete layouts/ directory with all required templates
  - Modern CSS with responsive design and clean styling
  - Proper theme documentation in README.md
- Update THEMES-HOWTO.md to accurately reflect theme behavior:
  - Clarify complete layout override (no fallback to defaults)
  - Document included minimal theme example
  - Correct layout processing documentation
- Update site.conf to use new minimal theme CSS path
- Ensure themes are complete packages when specified

themes/minimal/README.md

@@ -0,0 +1,49 @@
+# Minimal Theme for qsgen3
+
+A clean, minimal theme that demonstrates the proper qsgen3 theme structure.
+
+## Features
+
+- Clean, modern design
+- Responsive layout
+- Semantic HTML structure
+- Accessible typography
+- Fast loading
+
+## Structure
+
+```
+themes/minimal/
+├── layouts/                # Complete set of Pandoc templates
+│   ├── index.html         # Homepage layout
+│   ├── post.html          # Blog post layout
+│   ├── page.html          # Static page layout
+│   └── rss.xml            # RSS feed template
+└── static/                # Theme assets
+    └── css/
+        └── style.css      # Main theme stylesheet
+```
+
+## Usage
+
+1. Set in your `site.conf`:
+   ```
+   site_theme="minimal"
+   site_theme_css_file="css/style.css"
+   ```
+
+2. Run qsgen3:
+   ```bash
+   ./bin/qsgen3
+   ```
+
+## Customization
+
+- Edit `static/css/style.css` to modify the visual appearance
+- Modify layout templates in `layouts/` to change HTML structure
+- Add JavaScript files to `static/js/` and reference them in templates
+
+## Browser Support
+
+- Modern browsers (Chrome 90+, Firefox 88+, Safari 14+)
+- Graceful degradation for older browsers