img-retina mixin

Created by: MartijnCuppens

The img-retina mixin is not documented in the docs. https://github.com/twbs/bootstrap/blob/29f178d89129732d35ae812667709c64cdb28a6f/scss/mixins/_image.scss#L19-L36

Shall we:

• document this
• or remove the mixin (anno 2018 it might be better to use .svgs as background images)