docs: add stack labels example

[jlozovei]

Adding stack labels example, on Advanced > Custom Labels page, using a custom formatter to display stack series label instead of series value.

Found this issue to solve a problem, and it led me to a smoother solution which I think would be useful to others.

Working example of the formatter: https://codepen.io/jlozovei/details/NWJpYvy ✨

Sneak peek on the proposal