docs: update all gpgkey paths to the new url

PackageCloud automatically switched our key earlier this week but some docs were still referencing the old path.

Closes #3390

deb.mk

@@ -71,7 +71,7 @@ install-from-deb:
 	wget -nv -O - https://get.docker.com/ | sh
 
 	@echo "--> Installing dokku"
-	wget -nv -O - https://packagecloud.io/gpg.key | apt-key add -
+	wget -nv -O - https://packagecloud.io/dokku/dokku/gpgkey | apt-key add -
 	@echo "deb https://packagecloud.io/dokku/dokku/ubuntu/ $(shell lsb_release -cs 2> /dev/null || echo "trusty") main" | sudo tee /etc/apt/sources.list.d/dokku.list
 	sudo apt-get update -qq > /dev/null
 	sudo DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true apt-get install -yy dokku