Consolidate all moonweb sites into single Eleventy config

- Single eleventy.config.js at root with computed pathPrefix per section
- Accent colors inlined in base.njk (no per-site theme.css needed)
- Hub pages output at root via explicit permalinks
- Per-section sitemaps with tag-based collections
- .eleventyignore excludes stefankoelle, timecapsule, markdown files
- Deleted: 5 per-site configs, merge script
- Simplified: package.json (single build/dev commands)
- Simplified: GitHub Actions workflow (no matrix, no merge job)
- Fixed: parent links for sub-site detail pages

retro/404.html

@@ -1,9 +1,10 @@
 ---
 title: "Page Not Found"
 section: "retro"
+tags: "retro"
 description: "The page you are looking for does not exist."
 layout: base.njk
-permalink: /404.html
+permalink: /retro/404.html
 ---
 <div class="error-page">
   <h1>404</h1>