add other adsr examples to audio examples based on [vline~] and [slop~]

[porres]

I would keep the existing [adsr] envelope abstraction and would like to call it "linear" envelope, and just add another possibility in a 2nd abstraction with [slop~], see #226

I'd mostly only update example D02.adsr to include this other option, and I wouldn't really mess with other examples.

Also, it'd be nice to mention one can use [vline~] to have an attack and scheduled decay section.