feat: Implement robust theme system and document architecture

- Implement flexible theme switching via site.conf (site_theme, site_theme_css_file).
- Ensure correct copying of theme static assets, with theme assets overriding root assets.
- Resolve CSS linking issues by checking file existence after static copy and using correct paths for Pandoc.
- Refactor path construction to prevent duplication when using absolute/relative output paths.
- Create comprehensive how-it-works.md detailing system architecture, theme creation, and overall workflow.
- Clarify design philosophy: qsgen3 remains design-agnostic, only linking main theme CSS automatically.

static/css/style.css

@@ -0,0 +1,31 @@
+/* Basic styling - feel free to replace */
+body {
+    font-family: sans-serif;
+    line-height: 1.6;
+    margin: 20px;
+    background-color: #f4f4f4;
+    color: #333;
+}
+
+header h1 a {
+    color: #333;
+    text-decoration: none;
+}
+
+header p {
+    font-style: italic;
+    color: #666;
+}
+
+main {
+    background-color: #fff;
+    padding: 20px;
+    border-radius: 5px;
+}
+
+footer {
+    text-align: center;
+    margin-top: 20px;
+    font-size: 0.9em;
+    color: #777;
+}