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/cloud.tsx

@@ -22,7 +22,7 @@ type Status = "idle" | "submitting" | "success" | "error";
 
 function Cloud() {
   return (
-    <SiteShell>
+    <SiteShell width="default">
       <h1 className="text-3xl font-medium mb-3">Paseo Cloud</h1>
       <p className="text-white/70 leading-relaxed mb-10">
         For using Paseo across machines, with a team, or inside a company. Looking for design