Fix caddy installation docs (#5209)

Applies Caddy installations documentation fixes and also resolves the following issue when trying to install Caddy:

    W: GPG error: https://dl.cloudsmith.io/public/caddy/stable/deb/debian any-version InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ABA1F9B8875A6661

See:

 - caddyserver/website@930109e
 - caddyserver/website@2e255b1
 - caddyserver/website@0f4885e

Author: petarov

docs/guide.md

@@ -126,8 +126,8 @@ access code-server on an iPad or do not want to use SSH port forwarding.
 
 ```console
 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
+curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/gpg.key' | sudo apt-key add -
+curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/debian.deb.txt' | sudo tee /etc/apt/sources.list.d/caddy-stable.list
 sudo apt update
 sudo apt install caddy
 ```