Update Server installation instructions

Feedback from a user: Add instructions on how to install Server on other platforms (like Ubuntu). 

There is an `apt-get` command, so perhaps the issue is that the commands are not clearly separated by platform. Separating the docs in this way would make them more user-friendly, especially for newer users who may not already be familiar with the command-line.

https://puppet.com/docs/puppetserver/latest/install_from_packages.html