Rework docs navigation and move alternatives to top-level pages (#1487)

* Rework docs navigation and move alternatives to top-level pages

- Add nested docs nav with categories, collapsible groups, breadcrumbs,
  and a right-hand page outline with heading slugs.
- Move alternatives out of /docs/alternatives into /alternatives/*
  marketing pages with 301 redirects.
- Add github-slugger for heading IDs and require explicit width on
  SiteShell so new pages can't accidentally pick the narrow prose layout.
- Remove the best-practices doc from public-docs.

* Make docs content column prose-width

packages/website/src/server-entry.ts

@@ -33,6 +33,12 @@ export default {
       return Response.redirect(url.toString(), 301);
     }
 
+    const altRedirectMatch = url.pathname.match(/^\/docs\/alternatives\/(.+?)\/?$/);
+    if (altRedirectMatch) {
+      url.pathname = `/alternatives/${altRedirectMatch[1]}`;
+      return Response.redirect(url.toString(), 301);
+    }
+
     if (url.pathname === "/llms.txt") {
       return markdownResponse(buildLlmsTxt());
     }