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/llms.ts

@@ -1,3 +1,4 @@
+import { getAlternativePages } from "~/data/alternative-pages";
 import { AGENT_PAGES } from "~/data/agent-pages";
 import { type Doc, getDocs } from "~/docs";
 
@@ -27,17 +28,19 @@ function agentLine(agent: (typeof AGENT_PAGES)[number]): string {
   return `- [${agent.name}](${SITE_URL}/${agent.slug}): ${agent.subtitle}`;
 }
 
+function alternativeLine(page: ReturnType<typeof getAlternativePages>[number]): string {
+  const description = page.description.trim();
+  const suffix = description ? `: ${description}` : "";
+  return `- [${page.title}](${SITE_URL}${page.href})${suffix}`;
+}
+
 function topLevelDocs(): Doc[] {
   return getDocs().filter((d) => !d.slug.includes("/"));
 }
 
-function alternativeDocs(): Doc[] {
-  return getDocs().filter((d) => d.slug.startsWith("alternatives/"));
-}
-
 export function buildLlmsTxt(): string {
   const docs = topLevelDocs().map(docLine).join("\n");
-  const alternatives = alternativeDocs().map(docLine).join("\n");
+  const alternatives = getAlternativePages().map(alternativeLine).join("\n");
   const agents = AGENT_PAGES.map(agentLine).join("\n");
 
   return `${PRODUCT_PREAMBLE}