Clarify bundling guide: Configuring Your App section

[afoxman]

Discussed in #1556

For users coming in with a new react-native init app, the the "Configuring Your App" section doesn't make much sense. The feedback I got from this discussion is that there isn't a bundle command in the new RN app at all.

We need a clear path for RN init apps which are blank/new. Here and probably elsewhere. They need specific guidance -- in this case, probably "skip this section" or "make a new bundle script command and configure rnx-kit > bundle". Maybe under Configuring your App, create sections for New App and Existing App.