Remove brand colour mixin, include example
Remove brand colour mixin, include example

* Remove the `organisation-brand-colour` mixin which creates an
anti-pattern of needing to nest containers to achieve the desired affect
* Instead include a simpler example of how the list can be used.

alphagov/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit