Hide empty sidebar container on non-reference pages
Only render the sidebar <nav> element for pages within the reference/
hierarchy. Non-reference pages (blog, notebooks) no longer get the
empty 260px sidebar container. The JS also skips fetching sidebar.json
on non-reference pages, and the :empty CSS rule provides a fallback.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>