doc: Cordova JavaScript APIs (cordova.js)

**Issue Type**
[ ] Bug
[x] Feature Request

**Priority**
[ ] Minor
[x] Major
[ ] Critical
[ ] Blocker

**Environment**
* OS:
* Browser:

**Description**
It would be great if there was some text calling out that you need to include cordova.js and cordova-plugins.js in the main HTML file in order to get access to plugins and APIs, and that it makes the global cordova object available. The only indication that this needs to happen is that they're in the files produced by cordova create.

This caused some confusion for me, because I was integrating cordova into a web project, copied over my existing html file from the web side, and wasn't aware that those script tags needed to be in my file.

Start by locating the Cordova JavaScript API documentation and reviewing the cordova.js and cordova-plugins.js files mentioned in the issue. Add documentation explaining that both script tags are needed in the main HTML file for plugins and APIs, and that cordova.js exposes the global cordova object; done means this requirement is clear to someone integrating Cordova into an existing web project.

Written by the indexing model from the issue text.