Missing example implementation of HTML legend

[kurkle]

@etimberg @benmccann It has been a year and chartjs 3.0 is about to release. Any example or public API to power a cutom legend? I've come across a similar case like the one of @yusufozturk 's

Originally posted by @ace-han in #6887 (comment)

[etimberg]

We could show it as an example plugin in the docs. I think it should be relatively quick to implement. The APIs to do the legend hide/show are all public already 😄

[ace-han]

@etimberg I appreciate your quick response. Could you post some demo code here in this thread first, please?

Scrolling lots of legends in an extra div and being able to show/hide onClick is one of urgent tasks in my case.

Thx in advance.