Restructure Android Documentation

This issue is to keep track of a more complete documentation restructure that should be done after the PRs https://github.com/apache/cordova-android/pull/764 & https://github.com/apache/cordova-docs/pull/1009 is merged.

This is because the Android app bundling feature adds some new `build.json` features that isn't exactly related to signing, but the current `build.json` documentation is documented under `Signing the app`

The new structure at a high level should be as follows:

> ## building the app
> [details/information on building the app]
> ### Using build.json
> [shows table flags related to build options]
>
> ## signing the app
> [details/information on signing the app]
> ### Using build.json
> [shows table flags related to signing options...]

The original discussion can be found at https://github.com/apache/cordova-docs/pull/1009#issuecomment-512227371 and the mailing list discussion can be found [here](https://lists.apache.org/thread.html/b99006ec1c61dde0640688db39a34e8485bc1a56ca6537875df3ecf7@%3Cdev.cordova.apache.org%3E)

Review the Android app bundling changes in PRs 764 and 1009, along with the existing build.json documentation under “Signing the app.” Restructure the documentation into separate building and signing sections, each with its relevant build.json options, after those PRs are merged; the linked discussion and mailing-list thread provide the background.

Written by the indexing model from the issue text.