docs(aio): rework of the upgrade guide

This commit was worked on by a number of people including
@filipesilva, @gkalpak and @wardbell. It contains changes that:

* remove unused files,
* fix the bootstrap approach to ensure that bootstrap is in the correct Zone
* fix unclosed code-example tags
* replace use of "we" with "you"
* remove broken dual router example

Related to angular/angular.io#3541

aio/content/examples/forms/src/app/app.component.ts

@@ -1,8 +0,0 @@
-// #docregion
-import { Component } from '@angular/core';
-
-@Component({
-  selector: 'my-app',
-  template: '<hero-form></hero-form>'
-})
-export class AppComponent { }