Make step 3 on Heroku landing page clearer

Feedback from a user:

> Product offering seems clear to me but step 3 isn’t. I guess when it’s time to modify my own system a common concern could be if this implies on some significant refactor, so maybe I’d like more info on that? Other steps had more info for me to understand better what will happen or its limitations (for example, I really really like the clear list of which CI systems it works with), and then step 3 looked a bit nebulous in comparison.

I wonder if we need to provide more info on using Cloud Native Build packs and providing reference apps and instructions on how to instrument the app for cloud deployment?