[Docs] Add a user guide

[AVGP]

• Create a new docs/server-setup.md file
• Explain what Rendertron is good for (see the README)
• Explain the basic function of Rendertron (you can use information from the dynamic rendering guide
• Explain the basic setup. You can use this codelab as inspiration and link to configure and deploy to explain how to configure and deploy Rendertron.
• Explain that Rendertron does not have authentication, so it should not have access to internal systems and should optimally be behind a layer of authentication or a firewall.