Add Strapi CMS with Article and Category content types

Article is deliberately minimal: title, slug, Markdown body, cover image and
category. No Author content type — the administrator is the only writer.

The bootstrap hook grants the Public role find/findOne on the two content types
and disables self-registration, so the public API is read-only and the site has
no front-end accounts.

cms/public/robots.txt

@@ -0,0 +1,3 @@
+# To prevent search engines from seeing the site altogether, uncomment the next two lines:
+# User-Agent: *
+# Disallow: /