[docs] Improve "Compiling from the .sass files"

[machour]

https://github.com/yiisoft/yii2-bootstrap4/blob/master/docs/guide/assets-setup.md#compiling-from-the-sass-files

Here we're implying that in order to be able to modify bootstrap CSS, we should not install bootstrap from npm, and that we should maintain it manually instead.

The statement is obviously wrong, and I think that we might want to recommend a more modern approach, like using bootstrap from npm and properly customizing its variable in your site.scss file:

// Customize any bootstrap variable before including the main file from npm
// @see https://github.com/twbs/bootstrap/blob/v4.1.2/scss/_variables.scss

$primary: "#ffcc00"; 
@import "~node_modules/bootstrap/dist/bootstrap.scss"

.customer-area {
   // your own CSS
}

Would that be okay ?

[samdark]

Yes.