* 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
Adds the /grok SEO landing page (and its auto-generated route/sitemap
entries), the /agents directory card, and the Grok line in the ACP
catalog docs, following the Grok Build provider added to the in-app
catalog.
Every agent landing page now mentions both mobile and desktop, leads with
"open source", and uses a search-friendly hero title. Renames Pi to "Pi Agent"
and disambiguates other common-word names in titles.
