[doc] plugins: Add note about Heroku plugin sunset (#2508)
[doc] plugins: Add note about Heroku plugin sunset (#2508)

I didn't change more of the text, since the example plugin "heroku" was good at not being a built-in plugin, so the examples were better off keeping it, until a new non-built-in plugin has been added.
puma/puma,puma/puma,eileencodes/puma,eileencodes/puma,eileencodes/puma,puma/puma,puma/puma,eileencodes/puma