Display builds / artifacts more prominently

[JayFoxRox]

Builds are a big deal as they attract users. We should display them more prominently on the website.

The artifacts are currently available here: https://ci.appveyor.com/project/mborgerson/xqemu-c5j6o/build/artifacts

Basically when you open the website, there should probably be a "Download" button with a short rundown of the files the user needs (with links to Getting Started) and a warning that this is a development version, not a full release.

Our entire website, so far, feels like it's primarily aimed at very experienced users, with a lot of CLI, hex-editing, long command lines, ...

[mborgerson]

I added a section at the beginning of getting-started.md which has a direct-download link to the latest build from Appveyor via 14274c6. This isn't a big shiny download button, but it's an improvement. I've also moved the "Introduction" section into "Required Files" to more accurately reflect what the section is about. That said, we can keep this issue open as a reminder to that we need to improve the download experience.