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/components/alternative-page.tsx

@@ -0,0 +1,38 @@
+import { DocsMarkdown } from "~/components/docs-markdown";
+import { SiteShell } from "~/components/site-shell";
+import { getAlternativePage } from "~/data/alternative-pages";
+import { pageMeta } from "~/meta";
+
+export function alternativeRouteOptions(slug: string) {
+  const page = getAlternativePage(slug);
+
+  return {
+    head: () =>
+      pageMeta(
+        page?.title ?? "Alternative - Paseo",
+        page?.description ?? "",
+        `/alternatives/${slug}`,
+      ),
+    component: function AlternativePageRoute() {
+      return <AlternativePageContent slug={slug} />;
+    },
+  };
+}
+
+function AlternativePageContent({ slug }: { slug: string }) {
+  const page = getAlternativePage(slug);
+
+  if (!page) {
+    return (
+      <SiteShell width="default">
+        <p className="text-muted-foreground">Page not found.</p>
+      </SiteShell>
+    );
+  }
+
+  return (
+    <SiteShell width="default">
+      <DocsMarkdown>{page.content}</DocsMarkdown>
+    </SiteShell>
+  );
+}