doc/guide.md: Update caddy install instructions (#2601)

Caddy has new installation instructions for Debian.

Closes #2599

Author: nhooyr

doc/guide.md

@@ -180,8 +180,9 @@ Assuming you have been following the guide, edit your instance and checkmark the
 3. Install caddy https://caddyserver.com/docs/download#debian-ubuntu-raspbian.
 
 ```bash
-echo "deb [trusted=yes] https://apt.fury.io/caddy/ /" \
-    | sudo tee -a /etc/apt/sources.list.d/caddy-fury.list
+sudo apt install -y debian-keyring debian-archive-keyring apt-transport-https
+curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/cfg/gpg/gpg.155B6D79CA56EA34.key' | sudo apt-key add -
+curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/cfg/setup/config.deb.txt?distro=debian&version=any-version' | sudo tee -a /etc/apt/sources.list.d/caddy-stable.list
 sudo apt update
 sudo apt install caddy
 ```