$ sudo apt-get update
$ sudo apt-get install docbook-to-man 

docbook-to-man is installed in your system.

Make ensure the docbook-to-man package were installed using the commands given below,

$ sudo dpkg-query -l | grep docbook-to-man *

You will get with docbook-to-man package name, version, architecture and description in a table

docbook-to-man package basic information: