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

public-docs/updates.md

@@ -2,7 +2,8 @@
 title: Updates
 description: How Paseo releases work, the difference between stable and beta channels, and how to opt in to earlier updates.
 nav: Updates
-order: 18
+order: 5
+category: Getting started
 ---
 
 # Updates