Refactor SCSS imports to use @use syntax and improve organization

[jetzlstorfer]

Closes #80

[jetzlstorfer]

build output without warnings:

@jetzlstorfer ➜ /workspaces/cloudnativelinz.github.io (main) $ make run  
jekyll serve --livereload
Configuration file: /workspaces/cloudnativelinz.github.io/_config.yml
            Source: /workspaces/cloudnativelinz.github.io
       Destination: /workspaces/cloudnativelinz.github.io/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
       Jekyll Feed: Generating feed for posts
                    done in 0.3 seconds.
 Auto-regeneration: enabled for '/workspaces/cloudnativelinz.github.io'
LiveReload address: http://127.0.0.1:35729
    Server address: http://127.0.0.1:4000/
  Server running... press ctrl-c to stop.
        LiveReload: Browser connected
^C