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/rss.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
+<channel>
+  <title>$site_name$</title>
+  <link>$site_url$</link>
+  <description>$site_tagline$</description>
+  <language>en-us</language>
+  <lastBuildDate>$rfc_2822_date$</lastBuildDate>
+  <atom:link href="$site_url$/rss.xml" rel="self" type="application/rss+xml" />
+
+  $if(posts)$
+  $for(posts)$
+  <item>
+    <title>$it.post_title$</title>
+    <link>$it.post_url$</link>
+    <pubDate>$it.post_rfc_2822_date$</pubDate>
+    <guid isPermaLink="true">$it.post_url$</guid>
+    <description><![CDATA[$it.post_summary$]]></description>
+  </item>
+  $endfor$
+  $endif$
+
+</channel>
+</rss>