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/routes/sponsor.tsx

@@ -14,7 +14,7 @@ export const Route = createFileRoute("/sponsor")({
 
 function Sponsor() {
   return (
-    <SiteShell>
+    <SiteShell width="default">
       <h1 className="text-3xl font-medium tracking-tight mb-8">Sponsor</h1>
 
       {/* Founder note */}