docs: update advanced example

[tiendq]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update Advanced example is not working #154
• typo fix
• metadata update

Motivation / Use-Case

Update advanced example.

[alexander-akait]

How mode related to mini-css-extract-plugin?

[tiendq]

@evilebottnawi mode makes sense in the advanced example of this plugin (check README file), the example doesn't work if you use mode argument with webpack, what @EddyVinck used in #154. It works if NODE_ENV is used.

[michael-ciniawsky]

This is unrelated to mini-css-extract-plugin in particular and therefore unnecessarily 'bloats' the plugin README. The fact that webpack's mode doesn't rely on process.env.NODE_ENV needs to be extrapolated/documented elsewhere e.g in webpack/webpack.js.org