* 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
