(demo) add note on clipboard.js

[tlienart]

#680 for the steps.

also this uses clipboard 1.4 or something, current version is 2.0; this might not matter at all but might be worth pointing it out (it probably doesn't matter much).

Maybe a hover/image might be nice, something like https://doc.rust-lang.org/stable/book/ch07-02-defining-modules-to-control-scope-and-privacy.html but that could easily be done by the user

note: JuliaDocs/Documenter.jl#1456