Improve the visual presentation of the Rails Guides, bringing them more inline with the new marketing rubyonrails.org site, as part of a project with the Rails Foundation.

The CSS has been recreated from SCSS files, generated into the final pieces. The stylesrc directory contains those commented source files. The generate task will now process those scss => css files.

Co-authored-by: Carlos Antonio da Silva <carlosantoniodasilva@gmail.com>