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/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>