Add a stackblitz demo to docs

[ma2ciek]

A follow-up of the below issues:

• https://github.com/ckeditor/ckeditor5-angular/issues/122
• https://github.com/ckeditor/ckeditor5-angular/issues/146#issuecomment-545874446

It should be noted that the Stackblitz uses a different module resolution strategy and following the documentation won't work in this case.

[ma2ciek]

@mlewand, @Mgsy, @msamsel, do you know if have an official Stackblitz account? Or from which account I could create such a demo?