DRY the heroku documentation

[treaz]

Basically, Heroku requires a standalone deployement+some extra features.
In the Heroku docs, there is some duplication of the standalone docs (e.g. the JettyLauncher).
ideally, all this standalone information should just be in the standalone docs.

For the heroku doc, my proposal is to make a reference to the standalone docs and then add just the heroku specific information. And this I plan to do in the 2.7 docs only