Remove compiled css and js files from repository

• Removed files from all theme versions
• Added gitkeep files
• Updated readme file

Great job

Had problems with this one while working on #3368324 📌 Replace Classy core theme by contrib theme Fixed

Our gulp tasks require certain .css files to exists mainly

  'css:fonts',
  'css:mail',
  'css:editor'

When compiling you'll get the following error:
Error: File not found with singular glob: /rocketship_theme_starter/css/style.editor.css (if this was purposeful, use `allowEmpty` option)

We can either keep empty .css files or figure out how to change these tasks that they don't require the files to be present at first ( haven't looked into how we could do this )

Added empty css files to fix the first compilation issue

Automatically closed - issue fixed for 2 weeks with no activity.