21-DOT-DEV/websites ? reverse-engineered prompt

Build me a Swift based tooling project for the 21.dev websites.

I want it to generate and manage the static site assets for 21.dev and docs.21.dev, including sitemap.xml and robots.txt for each subdomain, with sitemap updates based on the site content and git last modified dates where that makes sense. It should also handle the Cloudflare header rules for production and preview builds, so HTML pages, static assets, downloads, and markdown files get the right cache and security headers in the right places.

Please wire it so the docs site can include DocC content and any nearby markdown files, and make sure the generated output is ready to deploy cleanly to Cloudflare. If there are any setup details or API credentials needed for sitemap submission, keep them documented in a simple way. Use current docs online if you need to confirm anything about Swift packages, Slipstream, or Tailwind.