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/security.md

@@ -2,7 +2,8 @@
 title: Security
 description: "Security model for Paseo: architecture overview, connection methods, relay encryption, and best practices."
 nav: Security
-order: 16
+order: 4
+category: Getting started
 ---
 
 # Security