docs(demos): nav push/pop api demo

demos/nav-push-pop/app.html

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+<ion-nav id="nav" [root]="rootPage" #content></ion-nav>
+<ion-overlay></ion-overlay>

demos/nav-push-pop/index.ts

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+import {App, Page, IonicApp, Config, Platform} from 'ionic/ionic';
+import {NavController, NavParams} from 'ionic/ionic';
+
+var PAGE_NUM = 2;
+
+@App({
+  templateUrl: 'app.html'
+})
+class ApiDemoApp {
+
+  constructor() {
+    this.rootPage = InitialPage;
+  }
+}
+
+@Page({
+  templateUrl: 'main.html'
+})
+export class InitialPage {
+  constructor(nav: NavController) {
+    this.nav = nav;
+  }
+
+  push() {
+    this.nav.push(Page2);
+  }
+}
+
+@Page({
+    templateUrl: "page-2.html"
+})
+export class Page2 {
+    constructor(
+        nav: NavController,
+    ) {
+        this.nav = nav;
+        this.pageNum = PAGE_NUM;
+    }
+
+    push() {
+      PAGE_NUM++;
+      this.nav.push(Page2);
+    }
+
+    pop() {
+      if (PAGE_NUM > 2) {
+        PAGE_NUM--;        
+      }
+      this.nav.pop();
+    }
+}
+

demos/nav-push-pop/main.html

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+<ion-navbar *navbar>
+  <ion-title>Nav Push/Pop</ion-title>
+</ion-navbar>
+
+<ion-content padding>
+  <ion-row>
+    <button full block (click)="push()">.push() new page</button>    
+  </ion-row>
+<ion-content>

demos/nav-push-pop/page-2.html

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+<ion-navbar *navbar>
+  <ion-title>Page {{pageNum}}</ion-title>
+</ion-navbar>
+
+<ion-content padding>
+  <ion-row>
+    <button full block (click)="push()">.push() another page</button>    
+  </ion-row>
+  <ion-row>
+    <button secondary full block (click)="pop()">.pop()</button>    
+  </ion-row>
+</ion-content>