Prior to this commit, generated Asciidoctor theme files were directly
committed in this project as static files.
This commit removes those files and instead builds on SASS files, just
like other Asciidoctor themes are generated (see
https://github.com/asciidoctor/asciidoctor-stylesheet-factory).
This commit also reorganizes the build to use a Gulp + NPM based build
for generating and optimizing those static resources before packaging
them.
Note that this change is resetting the whole Spring theme to a basic CSS
stylesheet - further commits are expected to get back to a proper theme.
Brian Clozel