You shuld check out Server Side Includes (SSI) which enables you to do inludes, run scripts etc via the webserver (Apache, Nginx). Caddy has its own version that also handles Mardown and Frontmatter. Caddys own documentation is using this feature in fact, and the source is available for those who want to see how it works. It's called "Templating" in Caddy, not SSI.