Root HTML layout for the Shadix docs site.
Links the locally-built Tailwind bundle (app.css, which also scans the
website/ templates via its @source) and the docs.js LiveSocket bundle, and
loads the PostHog analytics snippet (see posthog_snippet/0). The <body>
carries the shadix sandbox class plus the theme background/text so the
component styles and theme tokens apply to the whole page.