Document how to skin Core CKEditor

Pinged @mlewand from the CKEditor team with the question Is it possible to load a custom skin while still using D8's build of CKEditor that includes the moono skin? The answer to your question will depend on the CKEditor team's answer to my question :)

@Jeff Burnz Well, worst case, you could create your own custom build of CKEditor. See core/assets/vendor/ckeditor/build-config.js. Follow the instructions in there, and change only one thing. Change:

var CKBUILDER_CONFIG = {
	skin: 'moono',

to:

var CKBUILDER_CONFIG = {
	skin: 'the_awesome_skin_that_jeff_made',

… but that'll be a PITA to maintain. Could be a great short-term solution though.

config.skin option allows you to change the skin. If it's not in the skins directory, you could even provide an external path as a value, more on this in our config.skin documentation.

@mlewand came through already as you can see

I've updated https://www.drupal.org/developing/api/8/ckeditor with this information. See the documentation additions: https://www.drupal.org/node/2617470/revisions/view/9775399/9784155.

I do realize this is somewhat painful. If this is a frequent enough request, we can perhaps let themes specify

ckeditor_stylesheets:
  - css/base/elements.css
  - css/components/captions.css
  - css/components/table.css
  - css/components/text-formatted.css
ckeditor_skin: path/to/myskin

That'd be a new feature request though.