Update site build: remove sidebar from site pages, add universe step, redesign homepage
Site pages (blog, notebooks) don't need the reference sidebar — it's only
for @doc output. Also adds the opam universe build step to deploy-site.sh,
copies odoc.support into the site tree, and refreshes the homepage with
recent entries and an about section.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>