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.

content/posts/hello-world.md

@@ -0,0 +1,30 @@
+---
+title: "Hello, World!"
+date: "2024-05-30"
+author: "qsgen3"
+draft: false
+summary: "My first post using the qsgen3 static site generator."
+---
+
+Welcome to qsgen3!
+
+This is your first post. You can edit it or delete it and start writing your own content.
+
+## Markdown Features
+
+Qsgen3 supports standard Markdown features, including:
+
+- Headings
+- **Bold** and *italic* text
+- Lists:
+  - Unordered
+  - Ordered
+- [Links](https://example.com)
+- `Inline code`
+
+```bash
+# Code blocks
+echo "Hello from a code block!"
+```
+
+Enjoy creating your site!