Main document template that outputs a FreeMarker template for further processing

Adapted from https://github.com/asciidoctor/asciidoctor-backends/blob/e76aea86e7da174414a6d0c2e4d5553cd89bd226/erb/html5/document.html.erb
Got asciidoctor rendering working, but currently requires manually copying the asciidoc sources into the project
Tweaked alignment of gwt icon
Now downloading quick-start.asciidoc and tutorial.asciidoc from github during the build