Update widget-mixin.rst
Adding example coming from an old branch on sib-core not merged: see -> https://git.startinblox.com/framework/sib-core/compare/master...improve-documentation for reference.
Adding example coming from an old branch on sib-core not merged: see -> https://git.startinblox.com/framework/sib-core/compare/master...improve-documentation for reference.