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/site-shell.tsx

@@ -4,10 +4,10 @@ import { SiteHeader } from "~/components/site-header";
 
 interface SiteShellProps {
   children: ReactNode;
-  width?: "default" | "prose";
+  width: "default" | "prose";
 }
 
-export function SiteShell({ children, width = "default" }: SiteShellProps) {
+export function SiteShell({ children, width }: SiteShellProps) {
   const mainClasses =
     width === "prose" ? "max-w-prose p-6 md:p-12 mx-auto" : "max-w-5xl p-6 md:p-20 mx-auto";
   return (