Migrate website to Starlight with full-text resource search (#883)

* Add search functionality to Learning Hub index page

Add a client-side search bar that filters articles by title, description,
and tags. Sections with no matching results are hidden automatically.
Uses the existing .search-bar CSS pattern from the cookbook page.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* chore: remove deprecated layouts, theme script, and learning-hub config

Phase 5 cleanup of Starlight migration:
- Delete BaseLayout.astro (replaced by StarlightPage)
- Delete ArticleLayout.astro (replaced by Starlight docs rendering)
- Delete theme.ts (Starlight has built-in theme toggle)
- Delete src/config/learning-hub.ts (sidebar order now in astro.config.mjs)
- Replace learning-hub glob collection with Starlight docs collection in content.config.ts
- Keep search.ts (still used by homepage and all resource page scripts)

Build verified: 23 pages, no errors.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Migrate website to Starlight with full-text resource search

- Replace bespoke Astro layouts with Starlight integration
  - Homepage and resource pages use StarlightPage wrapper
  - Learning Hub articles rendered via Starlight docs collection
  - Starlight provides search, theme toggle, sidebar, ToC, a11y

- Add custom Pagefind integration for resource search
  - All 614 agents/skills/instructions/hooks/workflows/plugins
    indexed as custom records with deep-link URLs
  - Type filter pills (horizontal pill toggles) above results
  - Search results link directly to resource modals via #file= hash

- Move global.css to src/styles/ for Vite processing
  - Scope CSS reset to #main-content to avoid Starlight conflicts
  - Full-width page gradient via body:has(#main-content)
  - Light/dark theme support with Starlight gray scale inversion

- Delete old layouts (BaseLayout, ArticleLayout), theme.ts, config

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review feedback

- Fix pagefind-resources.ts header comment (pagefind:true not false)
- Remove unused base variable in cookbook/index.astro
- Replace hardcoded /awesome-copilot/ paths with relative links in index.md
- Delete stale public/styles/global.css (source of truth is src/styles/)
- Replace fragile getBasePath() with Astro config base in pagefind integration
- Document pagefind:true reasoning in astro.config.mjs
- Use proper visually-hidden pattern + :focus-visible ring for filter pills
- Remove dead header/nav/theme CSS from global.css (~160 lines)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/astro.config.mjs

@@ -1,12 +1,85 @@
 import sitemap from "@astrojs/sitemap";
+import starlight from "@astrojs/starlight";
 import { defineConfig } from "astro/config";
+import pagefindResources from "./src/integrations/pagefind-resources";
 
 // https://astro.build/config
 export default defineConfig({
   site: "https://github.github.com/",
   base: "/awesome-copilot/",
   output: "static",
-  integrations: [sitemap()],
+  integrations: [
+    starlight({
+      title: "Awesome GitHub Copilot",
+      social: [
+        {
+          icon: "github",
+          label: "GitHub",
+          href: "https://github.com/github/awesome-copilot",
+        },
+      ],
+      customCss: ["./src/styles/starlight-overrides.css", "./src/styles/global.css"],
+      editLink: {
+        baseUrl:
+          "https://github.com/github/awesome-copilot/edit/staged/website/",
+      },
+      sidebar: [
+        {
+          label: "Browse Resources",
+          items: [
+            { label: "Home", link: "/" },
+            { label: "Agents", link: "/agents/" },
+            { label: "Instructions", link: "/instructions/" },
+            { label: "Skills", link: "/skills/" },
+            { label: "Hooks", link: "/hooks/" },
+            { label: "Workflows", link: "/workflows/" },
+            { label: "Plugins", link: "/plugins/" },
+            { label: "Tools", link: "/tools/" },
+          ],
+        },
+        {
+          label: "Fundamentals",
+          items: [
+            "learning-hub/what-are-agents-skills-instructions",
+            "learning-hub/understanding-copilot-context",
+            "learning-hub/copilot-configuration-basics",
+            "learning-hub/defining-custom-instructions",
+            "learning-hub/creating-effective-skills",
+            "learning-hub/building-custom-agents",
+            "learning-hub/understanding-mcp-servers",
+            "learning-hub/automating-with-hooks",
+            "learning-hub/agentic-workflows",
+            "learning-hub/using-copilot-coding-agent",
+            "learning-hub/installing-and-using-plugins",
+            "learning-hub/before-after-customization-examples",
+          ],
+        },
+        {
+          label: "Reference",
+          items: ["learning-hub/github-copilot-terminology-glossary"],
+        },
+        {
+          label: "Hands-on",
+          items: [
+            {
+              label: "Cookbook",
+              link: "/learning-hub/cookbook/",
+            },
+          ],
+        },
+      ],
+      disable404Route: true,
+      // pagefind: true is required so Starlight renders the search UI.
+      // Our pagefindResources() integration overwrites the index after build.
+      pagefind: true,
+      tableOfContents: { minHeadingLevel: 2, maxHeadingLevel: 3 },
+      components: {
+        Head: "./src/components/Head.astro",
+      },
+    }),
+    sitemap(),
+    pagefindResources(),
+  ],
   redirects: {
     "/samples/": "/learning-hub/cookbook/",
   },