Provide an example of how to use module/no module

[KidkArolis]

By default, jetpack produces separate modern and legacy entry points, index.html and index.legacy.html. Since those files contain the runtime and embed entry points, it's not clear how one would go about module/no-module approach instead. I think this could possibly be achieved by customising the html template used. Try to see if it really works and provide an example in docs.