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/skills.md

@@ -2,7 +2,8 @@
 title: Orchestration skills
 description: "Paseo orchestration skills: teach coding agents to spawn, coordinate, and manage other agents using slash commands."
 nav: Skills
-order: 12
+order: 32
+category: Automation
 ---
 
 # Orchestration skills