Review VM preparation guide

[willow9886]

In an effort to steer people to specific content for their deployment platform of choice, we should consider simply adding applicable content from the VM pre-req guide to the OS-specific doc, as necessary.

That way, we could simply have a landing page with supported OS options, the user can click the OS of choice, then start reading and deploying the server.

https://gluu.org/docs/ce/4.0/installation-guide/