Add yearly archive pages

The year folders from the blog tutorial have no page of their own, so /articles/2025 is a 404. A template named index.jsx inside a bracketed folder fills them in, articles/[year]/index.jsx:

params(() => pages("articles/**").filter(p => p.date).map(p => ({ year: p.date.slice(0, 4) })));
data({ title: `Articles from ${page.params.year}` });
<ul>
  {pages(`articles/${page.params.year}`).reverse().map(p =>
    <li><a href={p.path}>{p.title}</a></li>
  )}
</ul>

Each generated year page takes over the year's grouping node and keeps its posts as subpages. Because these pages now have a source, pages("articles/**") includes them; streams that want only posts add .filter(p => !p.params).