Write detailed guide that covers how to install and manage gems

[mojavelinux]

To help new and returning users get up to speed quickly with the Asciidoctor ecosystem, we need a detailed guide that explains how to install and manage gems.

The guide should start with how to install Ruby (perhaps linking out to the guide proposed in #399). It should go on to explain how to:

• install a gem (gem install)
• install a prerelease gem (gem install with the --pre switch)
• check where it was installed (gem which)
• check that the bin script is on the path (command -v and PATH)
• update it when a new version is available (gem update)

The guide should also cover the difference between a global installation and how to manage gems inside a project (self-contained).

The guide should cover several gems, not just asciidoctor, since gems have different requirements.