Centralize robots.txt and sitemap

- One robots.txt at root referencing two sitemaps
- One central sitemap.xml with all pages (no per-site sitemaps)
- Deleted: 4 per-site sitemap templates, 4 per-site robots.txt files
- Simplified sitemap template (uses collections.all)

eleventy.config.js

@@ -27,16 +27,14 @@ module.exports = function (eleventyConfig) {
   // Root-level shared assets
   eleventyConfig.addPassthroughCopy({ "shared/base.css": "shared-base.css" });
   eleventyConfig.addPassthroughCopy({ "shared/favicon/hub.svg": "favicon.svg" });
+  eleventyConfig.addPassthroughCopy({ "hub/robots.txt": "robots.txt" });
 
-  // Per-section assets: favicon, robots.txt
+  // Per-section assets: favicon
   for (const [section, cfg] of Object.entries(SITES)) {
     const prefix = cfg.pathPrefix || ".";
     eleventyConfig.addPassthroughCopy({
       [`shared/favicon/${section}.svg`]: `${prefix}/favicon.svg`,
     });
-    eleventyConfig.addPassthroughCopy({
-      [`${section}/robots.txt`]: `${prefix}/robots.txt`,
-    });
   }
 
   // Hub: profile photo