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/docs-markdown.tsx

@@ -70,12 +70,14 @@ const docsMarkdownComponents: Components = {
 
 export function DocsMarkdown({ children }: { children: string }) {
   return (
-    <ReactMarkdown
-      remarkPlugins={docsRemarkPlugins}
-      rehypePlugins={docsRehypePlugins}
-      components={docsMarkdownComponents}
-    >
-      {children}
-    </ReactMarkdown>
+    <div className="docs-prose">
+      <ReactMarkdown
+        remarkPlugins={docsRemarkPlugins}
+        rehypePlugins={docsRehypePlugins}
+        components={docsMarkdownComponents}
+      >
+        {children}
+      </ReactMarkdown>
+    </div>
   );
 }