Document package.json fields in the reference

I can see how there is a reference page for `config.xml` but don’t see a corresponding reference page for `package.json`. I think the reference page for Cordova-specific `package.json` fields is needed.

I also think that how and when the Cordova CLI would update each of the fields in `package.json` is needed, just like this was already done for `config.xml`.

This issue is triggered by #1004 (deprecated plugins element in `config.xml`).

Start by comparing the existing config.xml reference page with the Cordova-specific fields in package.json and the documented Cordova CLI behavior. Document each relevant package.json field and explain how and when the CLI updates it, matching the coverage already provided for config.xml.

Written by the indexing model from the issue text.