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.

layouts/css/default.css

@@ -0,0 +1,110 @@
+/* Default Theme for qsgen3 */
+body {
+    font-family: sans-serif;
+    line-height: 1.6;
+    margin: 20px auto;
+    padding: 0 20px;
+    max-width: 800px;
+    background-color: #fdfdfd;
+    color: #333;
+}
+
+header {
+    background-color: #eee;
+    padding: 1em 0;
+    margin-bottom: 2em;
+    text-align: center;
+}
+
+header h1 a {
+    color: #333;
+    text-decoration: none;
+}
+
+header .tagline {
+    color: #555;
+    font-size: 0.9em;
+}
+
+nav ul {
+    list-style-type: none;
+    padding: 0;
+    text-align: center;
+}
+
+nav ul li {
+    display: inline;
+    margin-right: 15px;
+}
+
+nav ul li a {
+    text-decoration: none;
+    color: #007bff;
+}
+
+nav ul li a:hover {
+    text-decoration: underline;
+}
+
+article header {
+    background-color: transparent;
+    padding: 0;
+    margin-bottom: 1em;
+    text-align: left;
+}
+
+h1, h2, h3, h4, h5, h6 {
+    color: #333;
+    margin-top: 1.5em;
+    margin-bottom: 0.5em;
+}
+
+a {
+    color: #007bff;
+    text-decoration: none;
+}
+
+a:hover {
+    text-decoration: underline;
+}
+
+img {
+    max-width: 100%;
+    height: auto;
+}
+
+pre {
+    background-color: #f0f0f0;
+    padding: 15px;
+    overflow-x: auto;
+    border-radius: 4px;
+}
+
+code {
+    font-family: monospace;
+    background-color: #f0f0f0;
+    padding: 0.2em 0.4em;
+    border-radius: 3px;
+}
+
+pre code {
+    padding: 0;
+    background-color: transparent;
+}
+
+blockquote {
+    border-left: 4px solid #ccc;
+    padding-left: 15px;
+    margin-left: 0;
+    font-style: italic;
+    color: #555;
+}
+
+footer {
+    text-align: center;
+    margin-top: 3em;
+    padding-top: 1em;
+    border-top: 1px solid #eee;
+    font-size: 0.9em;
+    color: #777;
+}