Also I'm not suggesting you make it front and centre but as it is currently there's only one mention on your landing page and it's buried so far deep it might as well not be mentioned at all. I did also look through some of the other docs and still missed it. If I missed it you can bet others have too. All I'm suggesting is that it's worth being a little more explicit about that because that will be a key detail for some people.
You mention about it being written in Go. That seems less relevant (in my personal opinion) but that's a great place to add a line saying something like
"Caddy uses the ACME protocol to automatically manage secure certificates issued by Let's Encrypt."