content: add framework documentation for getting Ionic mode

[brandyscarney]

URL

https://ionicframework.com/docs

Issue Description

I can't find anywhere we document how to get the mode, for example in JavaScript:

const mode = Ionic.mode;

in Angular:

import { Config } from '@ionic/angular';

constructor(
  public config: Config
) { }
  
ngOnInit() {
  this.mode = this.config.get('mode');
}