build(aio): big move of docs related files (#14361)

All the docs related files (docs-app, doc-gen, content, etc)
are now to be found inside the `/aio` folder.

The related gulp tasks have been moved from the top level
gulp file to a new one inside the `/aio` folder.

The structure of the `/aio` folder now looks like:

```
/aio/
  build/         # gulp tasks
  content/       #MARKDOWN FILES for devguides, cheatsheet, etc
    devguides/
    cheatsheets/
  transforms/    #dgeni packages, templates, etc
  src/
    app/
    assets/
    content/    #HTML + JSON build artifacts produced by dgeni from /aio/content.
                #This dir is .gitignored-ed
  e2e/           #protractor tests for the doc viewer app
  node_modules/ #dependencies for both the doc viewer builds and the dgeni stuff
                #This dir is .gitignored-ed
  gulpfile.js   #Tasks for generating docs and building & deploying the doc viewer
```

Closes #14361

aio/src/app/app.component.spec.ts

@@ -0,0 +1,29 @@
+import { async, ComponentFixture, TestBed } from '@angular/core/testing';
+import { AppComponent } from './app.component';
+import { AppModule } from './app.module';
+
+import { NavEngine } from './nav-engine/nav-engine.service';
+
+describe('AppComponent', () => {
+  let component: AppComponent;
+  let fixture: ComponentFixture<AppComponent>;
+
+  beforeEach(async(() => {
+    TestBed.configureTestingModule({
+      imports: [ AppModule ],
+      providers: [
+        { provide: NavEngine, useValue: { currentDoc: undefined } }
+      ]
+    });
+    TestBed.compileComponents();
+  }));
+
+  beforeEach(() => {
+    fixture = TestBed.createComponent(AppComponent);
+    component = fixture.componentInstance;
+  });
+
+  it('should create', () => {
+    expect(component).toBeDefined();
+  });
+});