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/download.tsx

@@ -33,7 +33,7 @@ function Download() {
   const urls = downloadUrls(release);
 
   return (
-    <SiteShell>
+    <SiteShell width="default">
       <h1 className="text-3xl md:text-4xl font-semibold tracking-tight mb-2">Download</h1>
       <p className="text-muted-foreground mb-10">v{version}</p>