Docs/Vue.js/Vue CLI: Make it clear in the guide that using babel polyfills kills the performance

[oleq]

A follow-up of ckeditor/ckeditor5-vue#124.

In the discussion in the above issue, I found out that Vue CLI uses lots of (Babel) polyfills when building for production and that:

• increases the bundle size,
• dramatically impacts the already–not–so–perfect performance of the editor.

We need to clearly state in the guide that for the best performance, the build should be created using the --modern flag of Vue CLI.

[CKEditorBot]

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