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

package-lock.json

@@ -39223,6 +39223,7 @@
         "@tanstack/react-router": "^1.166.4",
         "@tanstack/react-start": "^1.166.4",
         "framer-motion": "^12.35.2",
+        "github-slugger": "^2.0.0",
         "lucide-react": "^1.7.0",
         "react": "^19.1.4",
         "react-dom": "^19.1.4",