Migrate preview and docs packages from Eleventy to Astro (#2694)

Co-authored-by: Bartek <xbartoszdobija@gmail.com>

preview/pages/markdown.astro

@@ -0,0 +1,11 @@
+---
+import RedirectLayout from '@shared/layouts/RedirectLayout.astro';
+
+// Front matter in preview/pages/markdown.html sets `redirect.to: prose.html`,
+// but shared/includes/redirect.html reads `page.redirect.to` — and in Eleventy
+// `page` is the built-in page object (no front matter), so the include gets an
+// empty url. The reference build (markdown.html) therefore redirects to the
+// relative root ".". We reproduce that exactly by passing no url.
+---
+
+<RedirectLayout />